RFR: 7903128: Overhaul jextract README

[Maurizio Cimadamore]

I've improved the README file, by putting the expanding the build section, and putting it more front and center.
I have also streamlined the remaining contents a little, and removed the parts that were describing the foreign API (not really needed here).

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Commit messages:
 - Overhaul README

Changes: https://git.openjdk.java.net/jextract/pull/3/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=3&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903128
  Stats: 116 lines in 1 file changed: 28 ins; 68 del; 20 mod
  Patch: https://git.openjdk.java.net/jextract/pull/3.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/3/head:pull/3

PR: https://git.openjdk.java.net/jextract/pull/3