[Flang] Fix formatting for FIRLangRef.html

Previously, FIRLangRef.md was incorrectly formatted.
This was due to how FIRLangRef.md had no page header,
and so the first entry would render incorrectly.

This patch introduces a header file, which is prepended to the FIRLangRef
 before it becomes a HTML file. The header is currently brief
but can be expanded upon at a later date if required.

This formatting fix also means the index page
can correctly generate a link to FIRLangRef.html and as such,
this patch also removes FIRLangRef from the sidebar and adds it to the main list of links.

Depends on D128650

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D129186