Add info about using Sphinx-Extensions "on the fly"

[marble]

https://typo3.slack.com/archives/C028JEPJL/p1637926085442500
"""
BTW, FYI:
It is nice to see that Sphinx Documentation has much support by others in the world.
Here somebody is showing a Sphinx extensions for interactively showing JSON data.
https://sphinx-data-viewer.readthedocs.io/en/latest/
But no, I don't think we should support this routinely in our DRC (Docker Rendering Container), because it doesn't seem wise to integrate all kinds of dependencies.
It is possible however, to use such an extension in local renderings with the DRC with proper parameters.
So this is also a reminder to myself, to document this in https://docs.typo3.org/m/typo3/T3DocsRenderingContainer/draft/en-us/
I'll create a ticket for myself there.
"""