RFR: 7903128: Overhaul jextract README [v7]

[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 three additional commits since the last revision:

 - Fix issue with libclang version
 - More table tweaks
 - Tweak table

-------------

Changes:
  - all: https://git.openjdk.java.net/jextract/pull/3/files
  - new: https://git.openjdk.java.net/jextract/pull/3/files/45c65328..a6435517

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=3&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=3&range=05-06

  Stats: 9 lines in 1 file changed: 0 ins; 0 del; 9 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