RFR: Add jextract guide

[Jorn Vernee]

On Tue, 9 Apr 2024 17:29:33 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

>> Add a comprehensive jextract guide under a new `doc/` folder.
>> 
>> This is meant as a comprehensive guide about the features of jextract and the code that it generates (including both examples from header files, the code that jextract generates, and corresponding Java user code). This is a first cut, and I'm anticipating quite a bit of comments.
>> 
>> Some sections of the readme have been moved to the guide (with minor edits here and there), and replaced by a link in the readme.
>
> doc/GUIDE.md line 3:
> 
>> 1: # Jextract Guide
>> 2: 
>> 3: The jextract tool parses header (.h) files of native libraries, and generates Java code,
> 
> Aren't .c files also viable for jextract?

Technically I don't think the extension matters, but C header files typically have the `.h` extension.

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1558067083