RFR: Add jextract guide

[Jorn Vernee]

Add a comprehensive jextract guide under a new `doc/` folder.

This is meant as a comprehensive guide about the features of jextract and the code that it generates (including both examples from header files, the code that jextract generates, and corresponding Java user code). This is a first cut, and I'm anticipating quite a bit of comments.

Some sections of the readme have been moved to the guide (with minor edits here and there), and replaced by a link in the readme.

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Commit messages:
 - Phrasing
 - Section of pre-precessor defines + other languages
 - Polish
 - Update README.md
 - Create GUIDE.md

Changes: https://git.openjdk.org/jextract/pull/231/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=231&range=00
  Stats: 995 lines in 2 files changed: 824 ins; 171 del; 0 mod
  Patch: https://git.openjdk.org/jextract/pull/231.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/231/head:pull/231

PR: https://git.openjdk.org/jextract/pull/231