RFR: Add jextract guide

[Maurizio Cimadamore]

On Wed, 10 Apr 2024 09:50:06 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 135:
> 
>> 133: ### Builtin Type Layouts
>> 134: 
>> 135: For every jextract run, regardless of the contents of the library header files, jextract
> 
> I'm not sure it's clear where these layouts definitions are introduced. I think the text should say that these are appended to the main header class.

Perhaps you can add a comment:

// mylib_h.java

as you do in later examples?

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559164739