Linking type annotations to the glossary

This should be linked to "unit-like" and vice versa. To the maximum extent that static typing allows, the two definitions should be the same.

_Originally posted by @nstarman in https://github.com/astropy/astropy/pull/18004#discussion_r2039880838_

Locate the glossary entry for “unit-like” and the related type-annotation definitions in the documentation. Check how each term is currently defined and linked, then make the two definitions match as far as static typing permits and ensure each links to the other.

Written by the indexing model from the issue text.