RFR: Add jextract guide [v12]

[Jorn Vernee]

On Tue, 16 Apr 2024 08:27:17 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> 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)

I think so. This is introductory, so someone may want to read up on the FFM API before diving deeper in  jextract

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1567320384