RFR: 7903128: Overhaul jextract README [v2]

[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 two additional commits since the last revision:

 - Simplify dir tree structure
 - More fixes

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

Changes:
  - all: https://git.openjdk.java.net/jextract/pull/3/files
  - new: https://git.openjdk.java.net/jextract/pull/3/files/8c0a2f9b..b2b97d16

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=3&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=3&range=00-01

  Stats: 22 lines in 1 file changed: 0 ins; 15 del; 7 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