Clean up and simplify READEME.md

• Remove duplicates
• Add badges for slack/discord
• Remove unmaintained community projects links
• Link to Coder registry
• link to modules, backstage-plugins and, envbuilder repos.
• @ericpaulsen @bpmct @kylecarbs @ammario add more suggestions

Probably:

• Sync README.md with https://coder.com/docs/v2/latest to prevent any drift.