RFR: Add jextract guide [v14]

[Jorn Vernee]

On Tue, 16 Apr 2024 13:36:35 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.
>
> 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

I've address the last of the review comments. I've also move all the javadoc links out of line, since they are so long (and it might be nice to have them in one place, should we need to update them in the future).

I'll integrate this PR if there are no further comments. Thanks for the detailed review!

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PR Comment: https://git.openjdk.org/jextract/pull/231#issuecomment-2059117294