Clean up equations and units in docs

[SouthEndMusic]

I am a sucker for neat mathematical notation (: One of my first PRs to the Ribasim repository was also like this.

These are most of the types of changes I made:

• Unified method for displaying units, without mixing $\TeX$ and non- $\TeX$
• Variable names and subscripts that consist of multiple letters are non-italic to indicate that they form one 'word' (I am not a fan of variable names that consist of multiple letters but I understand that it is unavoidable sometimes)
• Use proper symbols for partial derivatives

Some things still need to be fixed, I left some comments in a few places (most notably my question about what the unit t means).

I found DocumenterCitations.jl for a nicer incorporation of references, but given the plan to migrate the docs (#448) that might not be worth looking into.

[SouthEndMusic]

Merged into #448.