RFR: Add guide section about platform-dependent nature of jextract [v3]

[Maurizio Cimadamore]

On Thu, 18 Apr 2024 13:03:34 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> The current guide is lacking a discussion about the platform dependent nature of jextract, leaving users to wonder if and when the bindings generated by jextract can be used on different platforms.
>> 
>> This PR adds a couple of paragraphs to the guide to explain this.
>> 
>> Note: I've also fixed a typo in the README.
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add example of portable library practice

Marked as reviewed by mcimadamore (Reviewer).

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PR Review: https://git.openjdk.org/jextract/pull/237#pullrequestreview-2008849848