Update setup documentation

Following the setup instructions for this project's README, I found these (small) issues that impeded my minimal setup:

1. In README.md, we note in the documentation setup description that You might need to install mkdocsviapip, pipx, apt or the like. . In fact, we need the mkdocs, mkdocstrings, mkdocs-include-markdown-plugin, and mkdocstrings-python packages to serve the documentation. Reasonably we could switch this to please refer to requirements.txt for the necessary mkdocs packages.

2. The README.md says once the project has been made, test the filterset route to see if things are operational. The provided example of the request body has a trailing comma in the filter json data, and the mkdocs example body templates an array instead of a json object, which is the incorrect type.