Markdown formatting

[e-marshall]

Issue to keep track of formatting I want to update to make it easier to keep narrative text structured + accurate ie. heading names + section consistency ... :
[] Substitutions and variables for notebook sections (jb not recognizing substitution variables currently, still troubleshooting this)
[] Notebook introduction template
- want to ensure identical structure (intro, outline, learning goals) for all notebooks and have some cells set to collapsed by default
[] Might be useful to add a glossary (?)