RFR: Add jextract guide [v6]

[Maurizio Cimadamore]

On Mon, 15 Apr 2024 13:37:01 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> doc/GUIDE.md line 147:
>> 
>>> 145: some examples of how to use the generated Java code.
>>> 146: 
>>> 147: Most of the methods that jextract generates are `static`, and are designed to be imported
>> 
>> Uhm - I think the paras you moved fitted nicely in this introductory section
>
> I think they could go in either section? But, this way, we can discuss the `--header-class-name` option in the section about running, which I think makes more sense.

On a second look, it makes sense in this updated version.

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1565848813