### What is this PR for?
As more and more document pages are added, it's really hard to find specific pages. So I added searching feature to Zeppelin documentation site([jekyll](https://jekyllrb.com/) based site) using [lunr.js](http://lunrjs.com/).
- **How does it work?**
I created [`search_data.json`](6e02423f54/docs/search_data.json) which is used for docs info template. `lunr.js` combines all of the text from all of the docs in `docs/` into `_site/search_data.json`. It looks like below.
All the info are comes from [Jekyll YAML front matter](https://jekyllrb.com/docs/frontmatter/) variables. (i.e. title, group, description.. that's why I rewrote all docs' title and description.)
[search.js](6e02423f54/docs/assets/themes/zeppelin/js/search.js) will do this job using this data!
### What type of PR is it?
Improvement & Feature
### Todos
* [x] - Keep consistency for all docs pages' `Title`
* [x] - Add some overview sentences to all docs pages' `Description` section (this will be used as the result preview)
* [x] - Add apache license header to all docs page (some pages are missing the license header currently)
* [x] - Add LICENSE for `lunr.min.js`
### What is the Jira issue?
[ZEPPELIN-1219](https://issues.apache.org/jira/browse/ZEPPELIN-1219)
### How should this be tested?
1. Apply this patch and build `ZEPPELIN_HOME/docs` dir -> please see [docs/README.md#build-documentation](https://github.com/apache/zeppelin/tree/master/docs#build-documentation)
2. Click `search` icon in navbar and go to `search.html` page
3. Type anything you want to search in the search bar (i.e. type `python`, `spark`, `dynamic` ... )
### Screenshots (if appropriate)
### Questions:
* Does the licenses files need update? Yes, for `lunr.min.js`
* Is there breaking changes for older versions? no
* Does this needs documentation? no
Author: AhyoungRyu <fbdkdud93@hanmail.net>
Closes#1266 from AhyoungRyu/ZEPPELIN-1219 and squashes the following commits:
7ec8854 [AhyoungRyu] Modify 'no result' sentence
91b71a7 [AhyoungRyu] Remove Apache license header since JSON doesn't allow comment
34afd5d [AhyoungRyu] Add Apache license header to search_data.json
6784282 [AhyoungRyu] Minor search page UI update
0389d28 [AhyoungRyu] Make index.md not to be searched
9f1ba42 [AhyoungRyu] Disable enterkey press & change icon
bd4956a [AhyoungRyu] Add docs.js & search.js to exclude list in pom.xml
624b051 [AhyoungRyu] Add Apache license header to search.js
1381152 [AhyoungRyu] Fix search result skipping issue
6e775f5 [AhyoungRyu] Make pleasecontribute.md not to be searched
ee11136 [AhyoungRyu] Fix some typos
fa01299 [AhyoungRyu] Refine 'description' in some docs as @bzz suggested
da0cff9 [AhyoungRyu] Exclude lunr.min.js
36ba7f1 [AhyoungRyu] Add lunr.min.js license info
f6a05a6 [AhyoungRyu] Apply css style for the search results
68eb997 [AhyoungRyu] Attach 'Apache Zeppelin ZEPPELIN_VERSION Documentation: ' to title
d908c37 [AhyoungRyu] Add searching page
a951fa6 [AhyoungRyu] Add search icon to navbar
0688a79 [AhyoungRyu] Keep consistency all docs' front matter for the right search result
040f532 [AhyoungRyu] Add template for storing docs info based on jekyll front matter
0705bd6 [AhyoungRyu] Add js files: lunr.min.js & search.js
AhyoungRyu
