ETA: Following @timhoffm's suggestion, this pulls the tutorial content guidance from #26389 into this PR so it can be evaluated in conjunction with a rework of the index page.

In summary, this PR:

• provides a content guideline + sample inline with the diátaxis guideline as tutorial as guided walk through
• removes the links to the individual user guide documents because cross listing those documents as user guide pages and tutorials makes it very unclear what we consider a user guide versus a tutorial -> which hurts both:
  • discoverability -> it's unclear what sorta content is supposed to be on a page so hard to build intuition on what should be there
  • contributing -> it's unclear what kinda of contributions would be acceptable because the pages don't have a clear identity
• replaces the custom gallery code with a gallery listing. In theory we can just go back to a readme page, but this (uses an unreleased feature of sphinx so requires a release, am adding dev here just to demo what page would look like)
• moves the donut page to tutorials because it's showing a couple of things: how to make a donut chart and also how to annotate it - in an explanatory linear manner, talking the reader through each stage of the process. That's roughly the delineation between tutorial and example/how-to according to diataxes(https://diataxis.fr/tutorials/)