docs: add docs for mu / muD calculator

[yucongalicechen]

add docs for #281, #286, #219, #45

[cadenmyers13]

add docs for:

https://github.com/diffpy/diffpy.utils/blob/main/src/diffpy/utils/tools.py:

• compute_mud()
• compute_mu_using_xraydb()
• get_density_from_cloud() (not complete see feat: implement get_density_from_cloud() #333),

[cadenmyers13]

@sbillinge Not sure what is needed here. I parsed through the issues yucong linked and found the 3 functions listed above as "needing docs". It seems like these functions are already listed in the API docs (even though the last one isnt finished). Do you know what is needed here?

[sbillinge]

User docs are needed. Target audience is a chemist who wants to use the code to get a PDF from their lab data using PDFgetX3. These folks will never be looking at API docs, nor even using an API.

[cadenmyers13]

@sbillinge this can be closed too. completed via #375

[sbillinge]

closed as completed by #375