Integrated: Add jextract guide

[Jorn Vernee]

On Tue, 9 Apr 2024 17:20:22 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> 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.

This pull request has now been integrated.

Changeset: 7242f3e1
Author:    Jorn Vernee <jvernee at openjdk.org>
URL:       https://git.openjdk.org/jextract/commit/7242f3e1fd48fd74a5b1af486b9f3c8baba8f5e4
Stats:     1112 lines in 2 files changed: 941 ins; 171 del; 0 mod

Add jextract guide

Reviewed-by: mcimadamore

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