RFR: Add jextract guide

[Maurizio Cimadamore]

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 25:

> 23: ## Running Jextract
> 24: 
> 25: A native library typically has an `include` directory which contains all the header files

Here I wonder if instead of using pseudo name for paths, we should use some more real-looking names, like `/usr/include` and `/usr/lib`, which are common in many NIX systems.

doc/GUIDE.md line 28:

> 26: that define the interface of the library, with one 'main' header file. Let's say we have a
> 27: library called `mylib` stored at `/path/to/mylib` that has a directory `/path/to/mylib/include`
> 28: with all the header files. And let's say that we have a shell open in the root directory

Maybe instead of `with all the header files` we could expand to `where the header files of that library are stored`

doc/GUIDE.md line 29:

> 27: library called `mylib` stored at `/path/to/mylib` that has a directory `/path/to/mylib/include`
> 28: with all the header files. And let's say that we have a shell open in the root directory
> 29: of the Java project we're working on, which has an `src` source directory corresponding to

Suggestion:

of the Java project we're working on, which has a `src` source directory corresponding to

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559142532
PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559144738
PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559141069