RFR: Add jextract guide [v12]

[Jorn Vernee]

On Mon, 15 Apr 2024 23:01:03 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Update doc/GUIDE.md
>>   
>>   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>
>
> doc/GUIDE.md line 55:
> 
>> 53:   through `--output`)
>> 54: - `--library mylib` tells jextract that the generated bindings should load the library
>> 55:   called `mylib`. (The section on [library loading](#library-loading) discusses how is done)
> 
> "how is done" -> "how it's done."

This should be have "this is"

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1566596703