[Flang] Add a link from the docs html page to the FIR html page

The Fortran Language Reference is currently generated via tablegen,
however isn't present on flang.llvm.org/docs/

This patch adds FIRLangRef.md to the flang/docs directoy,
and adds a link to the generated HTML file in sidebar
under the 'Documentation' heading.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D128650