Add rst_migration_notes.txt that outlines RST formatting style.
Style fixups part 1: Tables, but headers also get fixed.

These style fixups will normalize the way certain files are
formatted so that they're mostly internally consistent with each
other.  It will go through several stages: tables, headers, toc,
sectnum, and link references.  It also goes without saying, but
the files have already mostly been fixed for greater readability.

Some tables that don't have header cells don't use the header
markup, but that's an obvious exception.

Content will not be updated, and this includes dangling links,
mainly because if the links are dangling, then they are in the
original docs. That should be fixed with general doc updating,
not now. I'm not even checking for unresolved links.

The old faq was also proofchecked, finally.
Style fixups part 2: Remaining headers: advancedtopics
Style fixups part 2: Remaining headers: maindir and corefilters