Use site variables (substitutions) throughout

[maneesha]

We have myst_substitutions set up in conf.py. This allows us to set variables for urls, email addresses, etc. and re-use them throughout the handbook. We should thoroughly go through each page and make sure it is using the substitution and not hard coding the url or email address. This will help us keep links and information up to date.

Marking this as "later" because it's not exactly broken, but should be addressed to help us maintain this resource.