RFR: 7903128: Overhaul jextract README [v4]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into readme-changes
 - Update README.md
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>
 - Update README.md
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>
 - Simplify dir tree structure
 - More fixes
 - Overhaul README

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

Changes:
  - all: https://git.openjdk.java.net/jextract/pull/3/files
  - new: https://git.openjdk.java.net/jextract/pull/3/files/e0f7e3bc..380f9a6b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=3&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=3&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 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