RFR: Add jextract guide [v6]

[Maurizio Cimadamore]

On Mon, 15 Apr 2024 13:41:29 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> doc/GUIDE.md line 869:
>> 
>>> 867:   WARNING: Skipping Foo (type Declared(Foo) is not supported)
>>> 868:   ```
>>> 869: 
>> 
>> Bitfields missing?
>> Also, support for primitive types > 64 bits.
>
> Bitfields is there. I'll add an item for types > 64 bits

I see bitfields now, but we should say something for e.g. `long double` on Linux

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1565824125