Update contribution guidelines

Content request

✏ Suggest ideas for new content!

Checklist

• This issue is not a duplicate, I've searched the Issue Tracker
• This issue is not a question, I'm aware that I can submit questions via Mastodon, Twitter, Email, Reddit or Matrix
• This issue is a content request, and not a bug report, feature request, or anything other

What are you missing or what could be improved?

1. Change landing page button to "Get Involved!"
2. Update contribution guide (mention UX contributions)

Do you have additional comments?

N/A