DOC: Standardize references to pandas in the documentation
In our documentation (files in `doc/source/../*.rst`), we refer to pandas as `Pandas`, `pandas`, and `*pandas*` (sphinx italics).

For documentation consistency, we should standardize these to one form. `pandas` is the preferable form in our documentation files.

Unfortunately I don't think we create a lint rule for this pattern since we have to be flexible to references and hyperlinks.