Replace Zulip in CONTRIBUTING.md with Discord

[pushfoo]

TL;DR: Match the doc repo in replacing Zulip with Discord

Changes

• Move Zulip to the bottom of the file as a historic location
• Add Discord Join instructions with links to channels
• Add cross-reference to Discord's limited forum channel documentation
• Various phrasing tweaks

[robinheghan]

Thank you! :)