create glossary page with common terminology and definitions

[chporter]

variables vs parameters vs initial conditions
i/o vs interface
exogenous vs endogenous
verification and validation

@platipodium @dawncassandra @gunnardressler

[chrstngyn]

@benbovy @chrstngyn

[platipodium]

Then let's add

• parameterization (in the sense of finding parameter values)
• parameterization (in the sense of formulating non-mechanistically the not represented processes)

[platipodium]

See my issue on adding discussion threads to all standards pages

[alee]

I think this is quite relevant with #189 as well. Will close #189 in favor of this one as it's a bit older and has some concrete examples