Add explanation page

[wtbarnes]

This would be a more long form page less about implementation and more about the details of the physics being done. These would not need to be overly detailed and could point toward the appropriate CHIANTI papers and technical reports but could still be a useful reference.

Some topics to cover in this section:

• [ ] Level populations equation: I think this should go here instead of the docstring because it is quite long and requires several equations. The docstring should then point to this page.
• [ ] Ionization equilibrium calculation: as above, describe system of equations being solved and how we solve them.
• [ ] Direct ionization rate derivation. Add what is in the doc string so the docstring can be shortened and simplified and make the derivation more complete.
• [ ] Optically-thin intensity for a given spectral line. This could also serve as a derivation for the contribution function (as implemented in fiasco) as well as the components that comprise it. This should be the first page as it essentially the starting point for all other components.

[wtbarnes]

It might be better to separate each of these into their own issue and then make this a project.