Make it easier to make sky plots

**Feature request**

Some plotting functionality is available only through `hats.inspection.plot_*(lsdb_catalog.hc_structure)`. It would be nice to make this functionality accessible directly from LSDB. Relevant documentation page would be very helpful.

- [ ] `plot_density` for unchanged catalog
- [ ] `plot_healpix_map` for `Catalog.skymap_histogram` output

**Before submitting**
Please check the following:

- [x] I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
- [ ] I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
- [ ] If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

Start by comparing the existing hats.inspection.plot_* functionality on lsdb_catalog.hc_structure with LSDB's Catalog.skymap_histogram output. Determine how plot_density and plot_healpix_map should be exposed directly from LSDB, then add the relevant documentation page. Done means both plotting paths are accessible through LSDB and documented.

Written by the indexing model from the issue text.