Create docs requirements files for reproducible docs builds

[ni-jfitzger]

Description of issue

Our original nimi-python ReadTheDocs project used the dependencies listed here, by default.

Newer projects use the latest stuff, which is a moving target. Since we've decided to replace our original project with new independent projects, we should lock down our docs build deps, so that our rendered documentation doesn't suddenly change on us.

[ni-jfitzger]

This would need to be balanced with a change to our release process to include periodically updating dependencies to tackle accumulated tech debt.