RFR: Add jextract guide [v5]

[Jorn Vernee]

On Fri, 12 Apr 2024 19:38:20 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Phrasing
>>   
>>   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>
>
> doc/GUIDE.md line 800:
> 
>> 798: 
>> 799: The value of these macros also affects the behavior of jextract. Therefore, jextract
>> 800: supports setting macro values on the command line using the `-D` or
> 
> Perhaps mentioning the distinctions between these macros and the ones described in the Constants section more specifically will be helpful?

Hmm, not sure what to say. There are no real distinctions, but jextract only generates an accessor if a macro is defined in the header files, not if it is only set on the command line with `-D`. (I'll add that at least)

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1565763762