update "Help and support"

[rootsmusic]

Checklist

• [X] I added a descriptive title
• [X] I searched open reports and couldn't find a duplicate

What happened?

Help and support should be updated to:

• [ ] Replace "Google Groups" (which was closed) with Discourse group.
• [ ] Add Matrix chat.
• [ ] "Report bugs" section should clarify that this repo "includes documentation that is common for conda and conda-build" and "The documentation for each of those individual repositories can be found in their respective repos".
• [ ] Clarify when to use conda's User Guide and when Anaconda Distribution's User Guide. (see comment)

### Tasks
- [ ] https://github.com/conda/conda-docs/issues/798