Use GitHub Discussions for periodic announcements

[ddbeck]

A couple of weeks ago, it came up on the WebDX call that we should have a dedicated announcements channel for web-features consumers. This is my proposal for doing that.

This PR adds documentation describing a process for publishing project announcements to GitHub Discussions.

If we choose to merge this PR, then we'll need to do the following just before merging:

• Turn on Discussions in the repository settings.
• Create a thread to post announcements to that is open to subscribing for notifications, but not open generally to replies (i.e., is locked)
• Turn off all other discussions (we don't really have a use for non-announcements yet).
• Update the documentation to link to the thread.

After merging, we should:

• Link to discussion thread and this PR's documentation in the next release's release notes.

[captainbrosset]

It would be nice to add this (or some version of it) to the repo's root-level README.md file too. Something along the lines of "To use web-features, do this ... you can also see releases here ... and follow this GH discussion for breaking and major changes announcements ...".

[ddbeck]

I added this to the package README with 8abe101. Since the repo README already links there, do you think that's sufficient?

[ddbeck]

This came up on the WebDX call today. One suggestion was to add a link to the announcements thread to our release notes boilerplate text. I'll update the Markdown file about this after the call.

[ddbeck]

OK, I this is ready now. If I get an approving review, I'll do the task list at the top, update the links, and merge this.