RFR: Add jextract guide [v12]

[Maurizio Cimadamore]

On Tue, 16 Apr 2024 08:45:12 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 682:
> 
>> 680: 
>> 681: - Certain C types bigger than 64 bits (e.g. `long double` on Linux).
>> 682: - Function-like macros (as mentioned in the [section on constants](#constants-macros--enums))
> 
> I wonder if here we got the reference backwards - e.g. perhaps the section on constants should just say that function-like macros are not supported and point here, then here we could say what workarounds could be used in more details?

Errno and endianness also come to mind in terms of unsupported stuff

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1567002714