gh-136672: Docs: Move Enum functions and add examples (GH-136791)

* Docs: Move Enum functions and add examples

When the `Enum` functions `_add_alias_` and `_add_value_alias_` were added in de6bca956432cc852a4a41e2a2cee9cdacd19f35, the documentation for them was done under `EnumType` instead of `Enum`.

This change moves them to the docs of the `Enum` class and adds an example for each function.

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Co-authored-by: Ethan Furman <ethan@stoneleaf.us>