DOC: Add See Also section to IndexSlice docstring regarding removing unused levels

[jschendel]

The behavior described in the Defined Levels section on the docs is often a source of confusion when slicing a MultiIndex.

Adding a "See Also" section to the docstring for IndexSlice that includes a link to Defined Levels section could be beneficial in giving this behavior visibility. A link to MultiIndex.remove_unused_levels in the new "See Also" section might also be helpful.

xref #21299