Reference docs for removed commands persist between builds

[medubelko]

Bug Description

Auto-generated pages for command references aren't tracked in Git or checked by gen_cli_docs.py.

I suggest we have the script check the list of command pages against the list of enabled commands, and remove any that are obsolete.

To Reproduce

1. Checkout Snapcraft 8.5.1 or earlier.
2. Build the docs.
3. Checkout the latest Snapcraft.
4. Build the docs.

The second build fails because the references for the outdated commands (disabled in #5195) are present and unindexed.

Relevant log output

Warning, treated as error:
/home/med/dev/snapcraft/docs/reference/commands/edit-registries.rst:document isn't included in any toctree

[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3884.

This message was autogenerated

[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3885.

This message was autogenerated