RFR: Add jextract guide [v12]

[Jorn Vernee]

On Tue, 16 Apr 2024 13:24:44 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> 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

let's save this for a followup, and get this stake in the ground first :) (it will probably also be easier to review a small section)

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1567361099