Docs: Include mention of `std::span` in the supported type conversions page

[parker-research]

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What version (or hash if on master) of pybind11 are you using?

Stable docs

Problem description

On this documentation page, there is no mention of std::span. However, it seems that std::span returned from a C++ function gets converted to a list in Python (which is great).

Would be even better if that was documented on this page: https://pybind11.readthedocs.io/en/stable/advanced/cast/overview.html#conversion-table

It also seems that the docstring/type annotations should indicate that the return type is a list.

Reproducible example code

Is this a regression? Put the last known working version here if it is.

Not a regression

[henryiii]

Docs contributions are always welcome. :)