Java: basic version of automodel extraction query docs

[kaeluka]

This adds documentation for the Java extraction queries.

Much of this content is writing down "lore" that accumulated over building and maintaining these queries.

I've tried to stick to information that I would find difficult to get across as comments in the ql/qll files.

Dear Reviewer

• I recommend not reviewing commit-by-commit, the documentation should make sense as a whole.
• I'm a bit reluctant to make the markdown file too big, because it's difficult to keep them in sync with the implementation: Let me know if there's any content missing 😄
• I'm trying not to depend on implementation details (that may change) unnecessarily - this would make the documentation prone to going stale quickly. If you spot locations where this obscures the message, please let me know.

[kaeluka]

@adityasharad: I've promised to ping you once I've written about why there are framework mode candidates that are both sink candidates as well as source candidates. The section called "Parameters as Source Candidates" tries to explain that. 🙂

@aeisenberg: I've also promised to ping you! 👋 👍

No review is required, but always welcome of course :)

[adityasharad]

Looks good!

[aeisenberg]

Nice. Mostly very minor copy-editing suggestions. My only bigger comment is around including a sentence or two describing what these queries are used for.

[kaeluka]

Thank you for the reviews, @adityasharad and @aeisenberg! I've responded 👌

[aeisenberg]

Thank you. Looks good.