RFR: Add jextract guide [v5]

[Nir Lisker]

On Fri, 12 Apr 2024 21:15:08 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> > Did you leave out "Bitfields" on purpose?
> 
> Jextract does not support bitfields (due to deficiencies in the libclang API). That's why there's no section for them :-)

Then maybe mention that :)

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PR Comment: https://git.openjdk.org/jextract/pull/231#issuecomment-2053511126