Fixes#31186. This commit adds more context about the behavior
of template reference variables in nested templates and moves
doc into concepts section.
PR Close#31195
This is part of a re-factor of template syntax and
structure. The first phase breaks out template syntax
into multiple documents. The second phase will be
a rewrite of each doc.
Specifically, this PR does the following:
- Breaks sections of the current template syntax document each into their own page.
- Corrects the links to and from these new pages.
- Adds template syntax subsection to the left side NAV which contains all the new pages.
- Adds the new files to pullapprove.
PR Close#36954
