Reorganize and improve documentation

[benjaminjkraft]

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 Figure out, and document, what semver means for us #63)
  • newer options for optional types
  • a bit more on custom scalars and integer types (fixes Document more clearly how to stringify large integers #128)

Reviewers, you can see what it all looks like together here.

Fixes #245.

I have:

• Written a clear PR title and description (above)
• Signed the Khan Academy CLA
• Added tests covering my changes, if applicable (n/a)
• Included a link to the issue fixed, if applicable
• Included documentation, for new features
• Added an entry to the changelog (n/a)

[StevenACoffman]

nice!