RFR: Add jextract guide [v4]

[Jorn Vernee]

On Wed, 10 Apr 2024 10:25:15 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Split sentence
>
> doc/GUIDE.md line 808:
> 
>> 806: ### Additional clang options
>> 807: 
>> 808: Users can also specify additional clang compiler options, by creating a file named
> 
> Maybe here we should start by saying that `jextract` uses clang to parse C files. And if additional options need to be passed to the clang instance used by jextract...

I've added a leading sentence here in the latest version: `Jextract uses an embedded clang compiler (through libclang) to parse header files.`

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1559525714