📝 Clarify and make obvious how to get help with C-PAC

[shnizzedy]

Related problem

The Troubleshooting and Help page says

C-PAC is beta software, which means that it is still under active development and has not yet undergone large-scale testing. As such, although we have done our best to ensure a stable pipeline, there may still be a few bugs that we did not catch.

If you find a bug, or would like to request a new feature please visit the GitHub Issues page for CPAC.

If you have a question that is not answered in the User Guide or encounter an issue not covered in the Common Issues and Solutions section below, you should submit it to the C-PAC user forum.

And the quickstart page says

Reporting errors and getting help

Please report errors on the C-PAC github page issue tracker. Please use the C-PAC google group for help using C-PAC and this application.

but there's no mention of help on the landing page, and Mattermost isn't mentioned anywhere in the docs

Proposed feature

Draft an unambiguous guide to getting user support, and add this guide or a link to it to some persistent element of the documentation (like the footer or sidebar).

Acceptance criteria

• Unambiguous guide to getting user support including
  • GitHub Issues
  • Google Group
  • C-PAC channel in Brainhack Mattermost workspace
  • Neurostars cpac tag
• Link to or transculsion of said documentation persists throughout user documentation

Alternatives

No response

Additional context

No response