Docs: Adding a pattern for reusing "Only on Read the Docs for Business" admonition (Diátaxis refactor)

[benjaoming]

In preparation for Diátaxis, we want an easy way to mix Explanation, How-tos and References so "Read the Docs for Business" features can be listed next to all other features, rather than a separate parallel structure.

The proposal here is quite simple:

• Use a DRY pattern for including the same admonition in all current RTD4B features
• We can always adjust the text later, once we see it in different contexts. I don't think we need to spend time on that now, as we cannot see how/where the admonition box will appear. But I could add an action item for Iteration 4.

Related: #9745

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--9754.org.readthedocs.build/en/9754/

• Developer's documentation (dev): https://dev--9754.org.readthedocs.build/en/9754/

[ericholscher]

Simple enough 👍