polish: Add show_aliases option + change where aliases appear

[ConnorGray]

This is a follow-up to the aliases feature generously started in #36 by @willemneal 🙂

Here I add a new option to control whether aliases are emitted, and also make small changes to the formatting:

1. Print command aliases only in their dedicated section, not in the subcommands listing of the parent command.
2. Move the command aliases out of the command header (simplifying the generated anchor link text) and into a bolded section underneath.