RFR: 7903635: jextract README page is obsolete

[Maurizio Cimadamore]

This PR updates the README with code snippets that are compatible with the new generated code.
When zooming in on what jextract generates to bind a function, instead of aiming for 100% fidelity, I tried to doctor the code a little, to make it a bit clearer, and avoid details such as holder classes and tracing support which, I think are too low-level at this stage.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/182/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=182&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903635
  Stats: 23 lines in 1 file changed: 0 ins; 10 del; 13 mod
  Patch: https://git.openjdk.org/jextract/pull/182.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/182/head:pull/182

PR: https://git.openjdk.org/jextract/pull/182