[docs][workflows] Finding a Source Plugin

[gillkyle]

Part of the Top 25 Learning Workflows initiative. See #13708 for the meta issue that this issue falls under.

User story

As a new Gatsby user, I want to find a source plugin for my CMS (or other data source) of choice.

(going to consider Contentful, WordPress, Sanity, and Postgres)

Evaluation

[Change emoji below based on your evaluation.]

Search	Discover	Complete	Linked	Tone	Style	Overall
😄	😐	😄	😄	😐	😄	😄

Steps taken to implement

1. Searchability
   1. searched gatsby with contentful; clicked second result, third result is the doc on sourcing 👍
   2. searched gatsby with wordpress; clicked first result
   3. searched gatsby with sanity; clicked first result, second result is also the sourcing doc
   4. searched gatsby with postgres; clicked first result
2. Discoverability
   1. searched wordpress Sourcing from WordPress is the first documentation result (same for contentful, sanity, postgres)
   2. searched database and got right to Sourcing from Databases page, which could link to even more plugins than it shows [rec] update the lists of plugins for sourcing information on pages like Sourcing From Databases to include as many plugins as we currently have (maybe this info could be pulled in dynamically sort of like the plugin library is?..)
   3. clickpath to sourcing content on .org (4 clicks)
      1. clicked on Docs in Navbar
      2. clicked on Reference Guides in Sidebar
      3. clicked on Sourcing Content and Data in Sidebar [rec] from Docs: add more information on finding a source plugin #13131 include an explanation on this overview page
      4. click on relevant page linked from overview (for Headless CMS this takes another 2 clicks)
3. Completeness
   • though there are a variety of plugins and each differs, the database section relies on plugin READMEs for steps (which aren't as big a focus), headless CMS guides have guides which is good
4. Linkedness
   • lots of links to examples and starters [rec] update list of Headless CMSs to include docs, cases, guides, and starters
     as noted in Docs: add more information on finding a source plugin #13131 the plugin tutorial links to itself and might have been moved from elsewhere [rec] fix linking to self in plugin authoring tutorial
5. Tone
   • tone is fairly neutral, maybe could sound more friendly?..
6. Style
   • 👍

Recommendations

• update the lists of plugins for sourcing information on pages like Sourcing From Databases to include as many plugins as we currently have (maybe this info could be pulled in dynamically sort of like the plugin library is?.. a little overkill for what's a pretty small list)
• from Docs: add more information on finding a source plugin #13131 include an explanation on this overview page
• update list of Headless CMSs to include docs, cases, guides, and starters
• fix linking to self in plugin authoring tutorial (this was addressed in a781ccb)
• update sidebar to include guides that have been written that aren't linked to (like Sourcing from Sanity that isn't included) (edit: a comment in the doc-links.yaml file shows that this list should only include the top 5 CMSs by usage, so just reorder them by usage)

[gillkyle]

Merged in #14846