RFR: Add jextract guide [v2]

[Jorn Vernee]

On Wed, 10 Apr 2024 09:58:01 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Apply suggestions from code review
>>   
>>   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>
>
> doc/GUIDE.md line 309:
> 
>> 307: ```
>> 308: 
>> 309: There's a getter and setter for each field of the struct (1), which takes a pointer to a struct
> 
> I see you didn't use bullet lists much, but I wonder if one would be helpful here instead of a single long para? Perhaps even a numbered list, where the number refers to the comment in the code?

I think a numbered list would be appropriate here

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559489693