RFR: 8231601: Update CONTRIBUTING.md to clarify process for contributing features plus Skara changes [v6]

[Kevin Rushforth]

> This PR updates the `CONTRIBUTING.md` guide to address the following:
> 
> 1. Clarify the process for adding new features / API changes, specifically that they must be discussed on the mailing list as the first step.
> 2. Add a link to the mailing list in the section regarding contributing bug fixes.
> 3. Remove the text about cross-linking the PR and JBS issue, and add a note that the Skara tooling takes care of this
> 4. Remove the section about manually resolving the JBS issue, and add a note that the Skara bot automatically does this when the PR is integrated.
> 5. Suggest the use of the "/reviewers 2" and "/csr" commands when appropriate
> 6. Update the note regarding which JDK(s) to use.

Kevin Rushforth has updated the pull request incrementally with one additional commit since the last revision:

  More review comments
  
  1. Renamed "Contributing to...codebase" section to "Coding style and testing guidelines", and switched that section with the "New features section"
  2. Updated recommend JDK
  3. Added forward reference for Draft and WIP PRs

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/303/files
  - new: https://git.openjdk.java.net/jfx/pull/303/files/47f51a3c..9d3820a0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=303&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=303&range=04-05

  Stats: 85 lines in 1 file changed: 40 ins; 38 del; 7 mod
  Patch: https://git.openjdk.java.net/jfx/pull/303.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/303/head:pull/303

PR: https://git.openjdk.java.net/jfx/pull/303