Type public Epochs, Evoked, and io members fully

[larsoner]

Okay this PR diff is big, but it's simple enough at its core:

1. Add a test that checks that our NumpyDoc-documented docstrings match the types we define in our code. This turned up a few minor issues (fixed).
2. Fill in type annotations for public interfaces in mne/io, mne/evoked.py, and mne/epochs.py, ensuring the test passes

I had Claude Opus 4.8 work on the test, but I simplified it and checked it. I also had Claude do the drudge-work of putting all those type annotations inline. The test ensures they're correct, though, so this should make review a bit easier (hopefully)!

This adds a little bit of work to future contributions having to type-define two places (docstring + code), but I don't think that's too onerous. Someday if numpy/numpydoc#196 (or numpy/numpydoc#601) lands we could maybe DRY things again.

I think with this scaffolding in place, we're safe to move to typing other modules in a similar fashion (or by using typestubs as a first pass, which @drammock is going to look into IIRC). In the meantime, I suggest we add this since it should make things slightly nicer for our end-users.

I considered pydoclint as a checker, but it is a static tool, so we need to get rid of %/docdict before we could pursue that.