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

[Jorn Vernee]

> 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

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Changes:
  - all: https://git.openjdk.org/jextract/pull/237/files
  - new: https://git.openjdk.org/jextract/pull/237/files/a5be8cbf..f9f28a4d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=237&range=02
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=237&range=01-02

  Stats: 8 lines in 1 file changed: 2 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jextract/pull/237.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/237/head:pull/237

PR: https://git.openjdk.org/jextract/pull/237