RFR: Add jextract guide

[Maurizio Cimadamore]

On Wed, 10 Apr 2024 13:26:10 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> 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`

whoops - true. I thought that row talked about manually writing a C wrapper around a C++ API.

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559443630