We've been in need of a documentation revamp for a while; a giant FAQ
was never the greatest structure and got worse as it grew. In this
commit I reorganize the documentation. Most of it is just moving around
existing text, but I added some new documentation here and there.

The changes:
- Many of the FAQ questions have moved to several new docs, on the
client/runtime, handling your schema, and various operation types; the
FAQ has answers to a few of the actually most frequent questions, as
well as a few things that didn't fit elsewhere.
- We now have a `docs/README.md` which acts as an index, so we can just
link to `/docs`.
- I lowercased the files that don't need match a GitHub convention, so
it's a bit less yell-y.
- I added documentation on:
  - how we version genqlient (fixes #63)
  - newer options for optional types
  - a bit more on custom scalars and integer types (fixes #128)

Reviewers, you can see what it all looks like together
[here](https://github.com/Khan/genqlient/tree/benkraft.docs-reorg/docs#readme).

Fixes #245.

I have:
- [x] Written a clear PR title and description (above)
- [x] Signed the [Khan Academy CLA](https://www.khanacademy.org/r/cla)
- [x] Added tests covering my changes, if applicable (n/a)
- [x] Included a link to the issue fixed, if applicable
- [x] Included documentation, for new features
- [x] Added an entry to the changelog (n/a)