RFR: Add jextract guide [v2]

[Maurizio Cimadamore]

On Wed, 10 Apr 2024 13:56:11 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> 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.

In that case, I think it would be clearer if we broke up the sentence. E.g. 


When using the --library <libspec> option, the generated code internally uses SymbolLookup::libraryLookup
to load libraries specified by `<libspec>`. If `<libspec>` denotes a library name, the name is then mapped to a platform dependent name using System::mapLibraryName.

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559494614