Improve web documentation and document styles

[choldgraf]

This PR has two focuses:

1. Adds documentation about how to customize style for the web, based on a recent conversation I had with @rowanc1
2. Moves our web docs to a new web/ folder and enables folder-based URLs for the MyST guide. I didn't do this for all docs because I first wanted to see how this looked to folks before suggesting we do it with other sections in a follow-up PR.

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[choldgraf]

@agoose77 I see a bunch of updates in here - can you merge this whenever you like? Alternatively I will merge this PR the next time I think about it if I don't hear otherwise.