Fixes #2238. In the docs, the examples that show the uv plugin don't work because uv doesn't get installed. This affects [Migrate from the Charm plugin to the uv plugin](https://documentation.ubuntu.com/charmcraft/latest/howto/migrate-plugins/charm-to-uv/) and the example at the end of [uv plugin](https://documentation.ubuntu.com/charmcraft/latest/reference/plugins/uv_plugin/#example). This PR: - Adds the `astral-uv` snap to the examples, as we agreed in [#2238](https://github.com/canonical/charmcraft/issues/2238). - Clarifies/expands the migration guide: - Adds [List the charm's dependencies](https://canonical-ubuntu-documentation-library--2413.com.readthedocs.build/charmcraft/2413/howto/migrate-plugins/charm-to-uv/#list-the-charm-s-dependencies) to clarify that direct dependencies go in `dependencies` in pyproject.toml. - Updates [List charm library dependencies](https://canonical-ubuntu-documentation-library--2413.com.readthedocs.build/charmcraft/2413/howto/migrate-plugins/charm-to-uv/#list-charm-library-dependencies) to recommend that dependencies from charmlib `PYDEPS` also go in `dependencies` (because they are runtime dependencies), as discussed in [#2312](https://github.com/canonical/charmcraft/pull/2312). I kept the dependency group approach as an alternative, but de-emphasized it. I also mentioned what to do for Python package libraries. - Adds [Lock the dependencies](https://canonical-ubuntu-documentation-library--2413.com.readthedocs.build/charmcraft/2413/howto/migrate-plugins/charm-to-uv/#lock-the-dependencies) to be explicit that uv.lock is needed.