Add append_css config variable to docs

[sneakers-the-rat]

Describe the bug

context
Hey! just trying this out for the first time. nice freaking tool y'all, exactly what i was looking for.

I was noticing that the notebooks were displaying oddly, and saw it was conflicting css from my sphinx theme. Was looking for a way to disable embedding additional styles in the rendered notebooks, and only found mention of the append_css option in the source, but not at https://myst-nb.readthedocs.io/en/latest/configuration.html

Don't have the time to check if this is a new feature or not, but just wanted to let you know it was what i needed and it should probably be in the docs <3

expectation
I expected append_css to be in the docs to occur.

bug
But instead append_css was not in the docs happens

problem
This is a problem for people doing looking for append_css to be in the docs because append_css was not in the docs.

Reproduce the bug

go to https://myst-nb.readthedocs.io/en/latest/configuration.html on the internet

List your environment

firefox!

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