Add sphinx docs source files

Add .rst source files for building documentation with
'sphinx'. The two mandatory files are conf.py containing
the build configuration and the master doc file index.rst.

Sphinx uses 'autodoc' to automatically include docstrings.
'autodoc' imports the modules and needs TUF installed in
the environment.

The following command will generate the documentation from the
source files in an html format:

`sphinx-build -b html docs/sphinx/source docs/sphinx/build/html`

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

docs/sphinx/source/conf.py

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+# Configuration file for the Sphinx documentation builder.
+#
+# This file only contains a selection of the most common options. For a full
+# list see the documentation:
+# https://www.sphinx-doc.org/en/master/usage/configuration.html
+
+# -- Path setup --------------------------------------------------------------
+
+# If extensions (or modules to document with autodoc) are in another directory,
+# add these directories to sys.path here. If the directory is relative to the
+# documentation root, use os.path.abspath to make it absolute, like shown here.
+#
+import os
+import sys
+sys.path.insert(0, os.path.abspath(os.path.join('..', '..', '..')))
+
+import tuf
+
+# -- Project information -----------------------------------------------------
+
+project = 'TUF'
+copyright = '2021, New York University and the TUF contributors'
+author = 'New York University and the TUF contributors'
+
+
+# -- General configuration ---------------------------------------------------
+
+master_doc = 'index'
+
+# Add any Sphinx extension module names here, as strings. They can be
+# extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
+# ones.
+extensions = ['sphinx.ext.napoleon']
+
+# Add any paths that contain templates here, relative to this directory.
+templates_path = ['_templates']
+
+# List of patterns, relative to source directory, that match files and
+# directories to ignore when looking for source files.
+# This pattern also affects html_static_path and html_extra_path.
+exclude_patterns = []
+
+
+# -- Options for HTML output -------------------------------------------------
+
+# The theme to use for HTML and HTML Help pages.  See the documentation for
+# a list of builtin themes.
+#
+html_theme = 'sphinx_rtd_theme'
+
+# Add any paths that contain custom static files (such as style sheets) here,
+# relative to this directory. They are copied after the builtin static files,
+# so a file named "default.css" will overwrite the builtin "default.css".
+#html_static_path = ['_static']
+
+autodoc_mock_imports = ['securesystemslib']
+

docs/sphinx/source/index.rst

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+Welcome to TUF's documentation!
+===============================
+
+This is the documentation of the **reference implementation** of
+`The Update Framework (TUF) <https://theupdateframework.github.io/>`_.
+
+The reference implementation strives to be a readable guide and demonstration
+for those working on implementing TUF in their own languages, environments, or
+update systems. It is written in Python and intended to conform to version 1.0 of the
+`TUF specification <https://theupdateframework.github.io/specification/latest/>`_.
+
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents:
+   
+   modern-implementation
+   legacy-implementation
+

docs/sphinx/source/legacy-implementation.rst

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+Legacy implementation
+=====================
+
+.. note:: The legacy implementation, with
+   `tuf/client/updater.py <https://github.com/theupdateframework/tuf/tree/develop/tuf/client/updater.py>`_ implementing the detailed
+   client workflow and `tuf/repository_tool.py <https://github.com/theupdateframework/tuf/tree/develop/tuf/repository_tool.py>`_
+   providing a high-level interface for repository operations, is in use in production systems, but is `no longer
+   being actively worked on <https://github.com/theupdateframework/tuf/tree/develop/docs/adr/0002-pre-1-0-deprecation-strategy.md>`_.
+
+

docs/sphinx/source/modern-implementation.rst

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+Modern implementation
+=====================
+
+The reference implementation is being refactored using  
+`modern Python <https://github.com/theupdateframework/tuf/tree/develop/docs/adr/0001-python-version-3-6-plus.md>`_
+to both:
+
+* Address scalability and integration issues identified in supporting integration
+  into the Python Package Index (PyPI), and other large-scale repositories.
+* Ensure maintainability of the project.
+
+This implementation consists of:
+
+* a "low-level" metadata API, designed to provide easy and safe access to
+  TUF metadata and handle (de)serialization from/to files, provided in the
+  :doc:`tuf.api` module.
+
+* an implementation of the detailed client workflow built on top of the
+  metadata API, provided in the :doc:`tuf.ngclient` module.
+
+.. note:: The modern implementation is not considered production ready and
+   does not yet provide any high-level support for implementing 
+   `repository operations <https://theupdateframework.github.io/specification/latest/#repository-operations>`_, 
+   though the addition of API to support them is planned.
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents:
+
+   tuf.api
+   tuf.ngclient

docs/sphinx/source/tuf.api.metadata.rst

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+Metadata API
+---------------------------------
+
+.. automodule:: tuf.api.metadata
+   :members:
+   :undoc-members:
+   :show-inheritance:

docs/sphinx/source/tuf.api.rst

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+Metadata API
+===============
+
+.. toctree::
+   :maxdepth: 4
+
+   tuf.api.serialization
+   tuf.api.metadata
+
+

docs/sphinx/source/tuf.api.serialization.rst

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+Serialization
+=============================
+
+JSON serialization
+-----------------------------
+
+.. automodule:: tuf.api.serialization.json
+   :members:
+   :undoc-members:
+   :show-inheritance:
+

docs/sphinx/source/tuf.ngclient.config.rst

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+Configuration
+=============
+
+.. automodule:: tuf.ngclient.config
+   :members:
+   :undoc-members:
+   :show-inheritance:
+

docs/sphinx/source/tuf.ngclient.fetcher.rst

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+Fetcher
+============
+
+.. automodule:: tuf.ngclient.fetcher
+   :members:
+   :undoc-members:
+   :show-inheritance:
+
+
+

docs/sphinx/source/tuf.ngclient.rst

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+Client API
+==========
+
+.. toctree::
+   :maxdepth: 4
+
+   tuf.ngclient.updater
+   tuf.ngclient.config
+   tuf.ngclient.fetcher
+

docs/sphinx/source/tuf.ngclient.updater.rst

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+Updater
+=========
+
+.. automodule:: tuf.ngclient.updater
+   :members:
+   :undoc-members:
+   :show-inheritance:
+