Remove noindex for docs

[SkafteNicki]

What does this PR do?

Fixes #865
Currently #1916 is blocked because we currently cannot reference our own metrics in our own docs due to the use of :noindex:. I propose that we remove the :noindex: from docs. The reason it is there as far as I am aware is to not have double entries when searching for something.

This is also true. Currently with :noindex: a search for BinaryAccuracy looks like

without :noindex: it would look like:

I think it would not be too destructive to have multiple of the same search results if we compare with the benefit that we can begin to actually reference our own metrics in the docs. This change also solves the linked issue, such that others can cross-reference torchmetrics in their own documentation.

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📚 Documentation preview 📚: https://torchmetrics--1998.org.readthedocs.build/en/1998/