website: make "deprecation" more visible: red boxes

[nschloe]

As discussed in #9475, numpy would likely profit from more visible deprecation notes than plain text in the docstring (see hstack's documentation, for example).

While actual deprecation is not desirable (for one, numpy 2.0 is a long way away), it was suggested to introduce big red boxes in the style of Python itself; see, e.g., here.

[eric-wieser]

Does .. deprecated: do the right thing here?

[nschloe]

The function binary_repr has .. deprecated:: 1.12.0 set, but I see nothing of that in the generated docs.

corrcoef actually shows something in the docs, but it isn't very visible either.

[njsmith]

It's fairly straightforward to redefine deprecated to be more visible, or to define a new directive, either by modifying the numpydoc plugin or by making a little numpy-local plugin.

[seberg]

The deprecated directive now uses red boxes in the rendered docs.