Docs reset: remove Cloud section + tutorials, add recipe examples

[cisaacstern]

This PR addresses item 2 of the tasks in #569 (comment):

Revert the nested hierarchy of the current docs (Pangeo Forge Recipes, and Pangeo Forge Cloud headings) in favor of making these docs "just" about pangeo-forge-recipes. The Cloud docs are now out-of-date, and doing this should make the final item easier, insofar as it impacts the overall narrative arc of the docs.

[rabernat]

I would consider adding a placeholder high-level section on "Deploying Recipes" after "Advanced Examples," even if it is just TODO for now.

[cisaacstern]

Getting close. Remaining TODO:

• Cleanup composition/examples/index
• [ ] Cleanup api_reference
• Cleanup contributing
• Flesh out deployment/cli

[cisaacstern]

This is a big overhaul the main objectives of which included:

• Remove obsolete Cloud docs
• Replace difficult-to-maintain tutorials with integration-tested recipe examples
• Restructure recipe docs to reflect Beam composition + current Deployment options

Additional wins here IMHO:

• Upgrading Sphinx by 3️⃣ major versions! Thinks look so contemporary in Sphinx 6

Going to merge this because there's a lot in the current docs that is stale which I want to get off the public internet ASAP.

There is still much room for improving the new sections here, as well as adding additional sections. Let's follow up on that in additional Issues/PRs. I am totally open to rethinking these docs based on everyone's feedback, just don't want to delay getting some updated info out into the public.

Tagging @rabernat @norlandrhagen @derekocallaghan for visibility.