Revisions part 2 of wiki: crosslinks + partial rewrite

As part of #324959 (closed), begin revisions of the wiki page, which needs a lot of love. I got partway through the work on Friday and realized it would make sense to land this work in two merge requests. In this merge request...

• Cross-link all the wiki API pages with each other, and with the main wiki page
• Begin bringing the page closer to CTRT (concept, tasks, resources, troubleshooting) format
• Remove outdated images that can be handled through text descriptions
• Begin rewriting subheadings for completeness and style
• The important part: reunite the marooned info about Group Wikis by moving it from the groups page to this page
• Fix a lot of cross-links

This page is NOT completely cleaned. However, I get nervous when I have a lot of cross-links, because the likelihood of conflicts rises. So let's land part of the work, and then come back for the rest.

Related issues

• Related to #324959 (closed)