doc/FAQ: convert to Markdown

It already was pretty close to Markdown, and maybe this way people will
be a little more likely to actually *read* the FAQ. Who knows?
doc/FAQ: rename to doc/FAQ.md
README.md: mention/link FAQ as well
doc/FAQ.md: update with s/Atheme/Shaltúre/, adjust SQL section

We're not fundamentally opposed to SQL support, but it comes with many
impracticalities. Thus it's not really a priority, but if we do find a
way to avoid these impracticalities, it might be implemented some day.

That said: the answer to the FAQ entry is "probably not".
doc/FAQ.md: address question on which release to use in production