Update docs to explain about uv and tox-uv

[dwilding]

In https://github.com/canonical/charmcraft/pull/2312 we're switching the kubernetes and machine Charmcraft profiles to use uv for building the charms. After this is released in Charmcraft, we should amend How to manage charms to:

• Mention that both uv and tox need to be installed in the local dev environment
• tox will need to be installed with uv tool install tox --with tox-uv
• Talk about updating uv.lock, which is required for correct packing (although uv add and uv remove automatically take care of this)
• Talk about when to use charmcraft clean?