RFR: Add jextract guide [v5]

[Nir Lisker]

On Mon, 15 Apr 2024 12:57:22 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> doc/GUIDE.md line 456:
>> 
>>> 454: try (Arena arena = Arena.ofConfined()) {
>>> 455:     MemorySegment cb = callback_t.allocate((a, b) -> a * b, arena);
>>> 456:     int result = call_me_back(cb);
>> 
>> Where is `call_me_back` defined?
>
> It's defined in the C file in the snippet above.

But we can't call the c function directly. Are there more java bindings that jextract generates?

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1565766202