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 14:

> 12: Java functions using `Linker::upcallStub`, access global variables through the addresses
> 13: returned by a lookup, and construct `MemoryLayout` instances for the structs they want to access.
> 14: Jextract aims to automate many of these steps, so that a client can instead immediately start

If you want to avoid capitalization of `jextract` you can say `The jextract tool...`

doc/GUIDE.md line 660:

> 658: ## Filtering
> 659: 
> 660: Some libraries are incredibly large (such as a platform SDK), and we might not be

Should we just say `windows.h` instead of the more vague "platform SDK" ?

doc/GUIDE.md line 665:

> 663: the elements we specify.
> 664: 
> 665: To allow for symbol filtering, `jextract` can generate a _dump_ of all the symbols

I noticed that in these sections, refrerences to `jextract` use a code block, whereas in the rest of the document they don't. I don't have a strong preference for one or the other, but we should make that consistent.

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559200246
PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559196211
PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559198874