[flang] Add new documentation main page
Add a new index page to be the Flang documentation mainpage instead of
Overview.md, which jumps straight into the compiler Design. The index file
needs to be in .rst format to use the toctree directive to create table of
Also use the sphinx_markdown_tables extension to generate html tables form
A number of additional style changes to the existing docs were needed to make
this work well:
- Convert all headings to the # style, which works better with toctree's titlesonly option. Ensure that there is only one top-level heading per document.
- Add a title to documents that don't have one for rendering on the index.
- Convert the grammar docs from .txt to .md. for better rendering
- Fixed broken link to a section in another document - sphinx does not seem to support anchor links in markdown files.
Depends on D87226
Reviewed By: sameeranjoshi
Differential Revision: https://reviews.llvm.org/D87242