Add a new page on model validation

[sanjayankur31]

• sciunit/neuronunit: http://sciunit.io/neuronunit.html

[pgleeson]

Maybe combine with page on testing... #48

[sanjayankur31]

[ ] test sciunit/neuronunit with latest python version (set up GH actions where appropriate)?
[ ] write up simple example like the tuner example showing sciunit + neuroml