Update mkdocs setup to use current release of mkdocs and associated packages.

Remove partial version dependencies in docs/requirements.txt that are currently broken per current README.md. Attempts to follow README.md and run mkdocs produced this error:

cannot find module 'materialx.emoji' (initialization failed)
  in "/Users/jalbers/git/PIH/openboxes/openboxes-OBPIH-6197/mkdocs.yml", line 105, column 20

[2]  + exit 1     mkdocs serve

This issue can be fixed if we specifiy 'materialx==0.0.0.dev1' in requirements.txt, but this then produces a page or two of deprecation warnings about a change in materialx.

-- doc/requirements.txt is updated to remove all revision specifications and use the latest releases of mkdocs and its dependencies.

-- mkdocs.yml is updated per the instructions in the deprecation warnings.

-- docs/README.md is updated to recommend use of a python virtual environment when creating the mkdocs config.