Docs: Fold all legacy "FAQ" content into the rest of the document

[jsquyres]

All the content in docs/faq/ is from the legacy FAQ (i.e., from https://www.open-mpi.org/faq/). It was trivially converted over to RST, but the content was largely left the same.

The intent is to audit all that content and either:

• Update it if necessary and fold it into other parts of the RST doc (thereby removing it from the docs/faq directory), or
• Remove it if is it out of date / no longer necessary

When this is done, the "FAQ" section should likely disappear from the RST docs.

[wckzhang]

Quincey is working on this issue and should be done soon as far as I am aware

[qkoziol]

PR is up: https://github.com/open-mpi/ompi/pull/11486

[qkoziol]

New PR, with correct signed-off-by and cleaner history: https://github.com/open-mpi/ompi/pull/11531