[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
contents.

Also use the sphinx_markdown_tables extension to generate html tables form
markdown.

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