RFR: Add jextract guide [v5]

[Jorn Vernee]

On Fri, 12 Apr 2024 19:39:26 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

> The first part is about running jextract, including a short review of an example, followed by an explanation about loading libraries. It looks like this section explains the arguments/options passed to the tool, but it skips the discussion about the macros option (appears much later in Preprocessor Definitions) and other options.
> 
> The second part is about the code that is generated, but then there is an explanation about `--header-class-name` and `--target-package`, which should be part of the previous part. I would expect that by the time the user gets to learning about the generated Java code, they would know what commands generated it.

Yes. The intent of this was to not bog down the intro with information that is not used that often, so people can get started quickly. But, in hindsight, I think it is better to just put all the discussion about command line arguments in 1 place. (Especially some of the info in the section about `-D` is important to get jextract to 'work'). I'll re-arrange the sections a bit.

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PR Comment: https://git.openjdk.org/jextract/pull/231#issuecomment-2056751066