RFR: Add jextract guide [v12]

[Maurizio Cimadamore]

On Mon, 15 Apr 2024 23:01:12 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 one additional commit since the last revision:
> 
>   Update doc/GUIDE.md
>   
>   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>

doc/GUIDE.md line 10:

> 8: by loading a native library (e.g., a `.so`/`.dll`/`.dylib` file), which is essentially an
> 9: archive of native functions and global variables. The user then has to look up the functions
> 10: they want to call using a `SymbolLookup`, and finally _link_ the functions by using the

There's some classes and methods referenced here - should we use links? (don't have strong opinion)

doc/GUIDE.md line 20:

> 18: The samples under the [`samples`](samples) direcotry are also a good source of examples.
> 19: 
> 20: Note that at this time, jextract only supports C header files. If you have a library written

should we say "jextract (and FFM) only support..." ?

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1566951897
PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1566953749