RFR: Add jextract guide [v5]

[Jorn Vernee]

On Mon, 15 Apr 2024 12:59:53 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> doc/GUIDE.md line 618:
>> 
>>> 616: 
>>> 617: C allows variable declarations to have an inline anonymous type. Jextract handles in
>>> 618: particular cases where a struct's field has an inline type specially. For instance, if we
>> 
>> "Jextract handles in particular cases where a struct's field has an inline type specially."
>> 
>> I don't understand this sentence. Does Jextract handle nested types only when a struct's field has an inline type?
>
> C doesn't allow other kinds of nested types, so yeah

I'll just remove the 'Jextract handles in particular cases where a struct's field has an inline type specially.' part here

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1565757187