RFR: Add jextract guide

[Jorn Vernee]

On Wed, 10 Apr 2024 10:26:28 GMT, Maurizio Cimadamore <mcimadamore 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.
>
> doc/GUIDE.md line 814:
> 
>> 812: ## Other Languages
>> 813: 
>> 814: As noted in the introduction, jextract currently only supports C header files, but many
> 
> Should we say something about many libraries (e.g. libclang) providing a C interop wrapper around their C++ impl?

That's in the table below: `many C++ libraries have a C interface`

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

PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559434905