RFR: Add jextract guide [v14]

[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.

Jorn Vernee has updated the pull request incrementally with three additional commits since the last revision:

 - mor review comments
 - out-of-line javadoc links
 - more review comments

-------------

Changes:
  - all: https://git.openjdk.org/jextract/pull/231/files
  - new: https://git.openjdk.org/jextract/pull/231/files/b5ab52c9..c3242781

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=231&range=13
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=231&range=12-13

  Stats: 99 lines in 2 files changed: 37 ins; 8 del; 54 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