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

[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.

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

Commit messages:
 - Address review comments
 - Address review comments + additional fixes
 - 8231601: Update CONTRIBUTING.md to clarify process for contributing features plus Skara changes

Changes: https://git.openjdk.java.net/jfx/pull/303/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=303&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8231601
  Stats: 100 lines in 1 file changed: 50 ins; 25 del; 25 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