Formatting issues with nested lists

[balhoff]

Comparing this source file (GitHub rendered):

https://github.com/OBOAcademy/obook/blob/master/docs/explanation/taxon-constraints-explainer.md

with the mkdocs rendered version:

https://oboacademy.github.io/obook/explanation/taxon-constraints-explainer/

There are a bunch of problems with rendering nested bullets and nested paragraphs in the mkdocs version. Example:

vs.

[balhoff]

Apparently I need to use 4 spaces: https://github.com/Python-Markdown/markdown/issues/3

I'll try updating the source file.