RFR: Add jextract guide

[Nir Lisker]

On Tue, 9 Apr 2024 17:20:22 GMT, Jorn Vernee <jvernee 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?

doc/GUIDE.md line 4:

> 2: 
> 3: The jextract tool parses header (.h) files of native libraries, and generates Java code,
> 4: called 'bindings', which  use the Foreign Function and Memory API (FFM API) under the hood,

Double space after `which`.

doc/GUIDE.md line 8:

> 6: 
> 7: Interacting with native C code through the FFM API works
> 8: by loading a native library (e.g. a `.so`/`.dll`/`.dylib` file), which is essentially an

Comma after `e.g.`.

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1558058247
PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1558058784
PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1558060749