Merge /support/chat and /contribute/chat into /about/contact

While working on /about/contact we pointed to /support/chat and /contribute/chat.

Actually these pages are now linked almost only from /about/contact and could be merged into it. While doing so we could improve them a bit, for example by factorizing the duplicated content.

Also consider what to do with /support/talk which duplicates this info again.

Related issues

• Related to #7698 (closed)
• Related to #11515
• Related to #15130
• Blocked by #11424 (closed)

Original created by @sajolida on 11423 (Redmine)