General updates to various wiki pages based on issues noted over the past few months.

[seanlip]

Summary of fixes:

• Deprioritize oppia-dev@ in favour of GitHub Discussions
• Add a suggestion to include debugging info with Beam jobs
• Ask new contributors to focus only on "Impact: High/Medium" issues, and to avoid "triage needed" issues.
• Move developer workflow team to the top of the list of teams, since the other teams tend to have more applicants
• Ask new contributors to actively work to get their PRs in once they're assigned. Add advice about writing a debugging doc.
• Add more information about how to use debugging docs.
• Ask contributors to file a flake report if they encounter a flake, and to link to it if requesting a restart.
• Modify the "launch new feature" instructions to ask contributors to fill out the form when their PR is close to being merged, instead of after it's merged; this is based on discussion with the server admins who would appreciate a heads-up.
• Ask contributors to verify the changes in the "Files Changed" tab before requesting review.
• Clean up the sidebar a bit.

[Ash-2k3]

Thanks @seanlip, left some comments, PTAL.

[seanlip]

@U8NWXD @Ash-2k3 PTAL, I made edits. Thanks!

[Ash-2k3]

Thanks Sean, just one more comment from me.

[seanlip]

Thanks @Ash-2k3 -- replied.

[Ash-2k3]

LGTM, Thanks! @seanlip