RFR: Add jextract guide [v12]

[Jorn Vernee]

On Tue, 16 Apr 2024 08:55:14 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 676:
> 
>> 674: In the above snippet, note that the load of the `baz` field value on the last line will
>> 675: _see_ the update to the `bar` field of the `foo` instance on the line before.
>> 676: 
> 
> Perhaps we could also add a section (maybe in a followup PR) on fields/globals with array types, as jextract generates extra metadata (e.g. dimensions) and the Java type used for these accesses is MemorySegment (with bulk copy support on `set`)

Yes, good point. This was missed

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1567357664