These are several fixes for the flang site. The look has been changed to
match clang since flang, like clang, is a frontend. Some broken links
were removed. Most fixes are to secton titles so the table of contents
is generated correctly. A minor typo has been fixed.

Recommonmark has been deprecated, then archived last year. This was
tracked by: https://github.com/llvm/llvm-iwg/issues/30

See https://github.com/readthedocs/recommonmark

This patch migrates all our doc to use myst

Additional details for bot maintainers: https://discourse.llvm.org/t/maintenance-required-on-sphinx-build-bots/73612

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D135570

I went over the output of the following mess of a command:

`(ulimit -m 2000000; ulimit -v 2000000; git ls-files -z | parallel --xargs -0 cat | aspell list --mode=none --ignore-case | grep -E '^[A-Za-z][a-z]*$' | sort | uniq -c | sort -n | grep -vE '.{25}' | aspell pipe -W3 | grep : | cut -d' ' -f2 | less)`

and proceeded to spend a few days looking at it to find probable typos
and fixed a few hundred of them in all of the llvm project (note, the
ones I found are not anywhere near all of them, but it seems like a
good start).

Reviewed By: awarzynski, clementval

Differential Revision: https://reviews.llvm.org/D130844

This patch adds documentation on FIR array operations
and their usage.

Reviewed By: schweitz

Differential Revision: https://reviews.llvm.org/D115077