RFR: 7903128: Overhaul jextract README [v5]

[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).

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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Changes:
  - all: https://git.openjdk.java.net/jextract/pull/3/files
  - new: https://git.openjdk.java.net/jextract/pull/3/files/380f9a6b..7092d796

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=3&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=3&range=03-04

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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