RFR: Add jextract guide [v3]

[Jorn Vernee]

On Wed, 10 Apr 2024 10:10:28 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   review comments
>
> doc/GUIDE.md line 486:
> 
>> 484: `callback_t` instance.
>> 485: 
>> 486: Jextract generates function pointer classes like the `callback_t` class for function
> 
> I know what you mean here - but I wonder if readers will get it w/o some kind of example.

I'll add some examples as well.

> doc/GUIDE.md line 665:
> 
>> 663: the elements we specify.
>> 664: 
>> 665: To allow for symbol filtering, `jextract` can generate a _dump_ of all the symbols
> 
> I noticed that in these sections, refrerences to `jextract` use a code block, whereas in the rest of the document they don't. I don't have a strong preference for one or the other, but we should make that consistent.

I'll just switch to the plain jextract, as that's a little easier on the eyes. (This section was copied from the readme)

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559507298
PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559509062