Docs: Refactor all business features into feature reference + change "privacy level" page (Diátaxis)

[benjaoming]

This refactor does the following:

• Empties and removes the "Read the Docs for Business" navigation
• Moves all existing business features into the common features section
• The former "business feature index" is now an "About Read the Docs for Business" page that attempts to write a simple reference, which I think we can afford to be more technical in the future. The new "Reference / About" section will fit this content.
• Moves "/commercial/subscriptions" to how-tos
• Yes, I did a diagram that's very simplified, but it's the image that I'd want people to have in their head when they think about the relationship between RTD4B and RTD4C.

Open questions:

• How do we refer to "landing pages" or "marketing website"? What's the word that we would use in our documentation? "Website" is the best term moving forwards, since we're unifying various landing page experience 👍
• Should this PR also just jump straight into refactoring the old "About" navigation section into reference? Not seeing any blockers... Doing remaining work in a separate PR because we might want to go into details with the About Read the Docs for Business page

Refs: #9746

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--10007.org.readthedocs.build/en/10007/

• Developer's documentation (dev): https://dev--10007.org.readthedocs.build/en/10007/

[benjaoming]

@ericholscher I'm gonna have a look at the other review feedback you've provided. Would you be able to have a quick look here at the 2 "open questions" in the description ⬆️ ?

[benjaoming]

Went over the open questions, and this PR is now open for review 🎉

[ericholscher]

This is great! 💯 Lots of nitpicks, as usual :) Been hanging out with writers for too long.

[ericholscher]

Does this work? I never know if it's going to expand first or not.

[benjaoming]

This one worked, but we can't write *|com_brand|* 🤦

[ericholscher]

Yea... could be a good use case for myst, but not sure if they have substitutions heh :) There's a hack to get nested syntax, I think using another substitution or something janky.

[benjaoming]

Seems everything is good! I'll leave a comment thread unresolved in case needs attention, but doesn't seem critical to me.

[benjaoming]

Nice to get this in wrt other PRs on feature reference and TOC refactoring.