RFR: Add jextract guide [v2]

[Maurizio Cimadamore]

On Wed, 10 Apr 2024 13:33:36 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> doc/GUIDE.md line 68:
>> 
>>> 66: 
>>> 67: When using the `--library <libspec>` option, the generated code internally uses [`SymbolLookup::libraryLookup`](https://docs.oracle.com/en/java/javase/22/docs/api/java.base/java/lang/foreign/SymbolLookup.html#libraryLookup(java.nio.file.Path,java.lang.foreign.Arena))
>>> 68: to load libraries specified by `<libspec>`, after potentially mapping the name of the library to a platform dependent name using [`System::mapLibraryName`](https://docs.oracle.com/en/java/javase/22/docs/api/java.base/java/lang/System.html#mapLibraryName(java.lang.String)).
>> 
>> potentially? Are there cases where mapping a library name is a no-op?
>
> The `<libspec>` might be a path (using `:`), in which case we don't  do any mapping, right? I don't mind removing the `potentially` here though.

ah I see. I mistakenly read `<libspec>` as if it was a library name.

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559489798