[BUG] nbdev_install_hooks not installing nbdev_clean hook

The `nbdev_install_hooks`, contrary to the documentation and expectations, do not install `nbdev_clean` hook: only `nbdev_merge` (in .gitattributes and .gitconfig) and `nbdev_trust` (in .git/hooks/post-merge).

Because the [Git-Friendly Jupyter](https://nbdev.fast.ai/tutorials/git_friendly_jupyter.html) does not provide _any_ information about what this command should do, and reference documentation is next to useless in that regard, I am not able to fix the issue myself.

I use nbdev version 2.3.12

Start at the nbdev_install_hooks entry point and compare its behavior with the Git-Friendly Jupyter documentation. Inspect .gitattributes, .gitconfig, and .git/hooks/post-merge, then verify that running the command also installs the nbdev_clean hook and that the documentation explains the expected hooks.

Written by the indexing model from the issue text.