Feature: Add section in the docs for setting up cross section libraries

[yardasol]

Background and motivation
For all the details, see the discussion in #76 as well as issues referenced there.

TL;DR, we currently have no instructions or references in the docs regarding cross section library setup.

Description of idea
We should add to a section to the docs that give instructions -- or at the very least, provide references for instructions -- on setting up cross section libraries for each supported depletion code.

Implementation details

• Add a new .rst file or modify installation.rst under docs/ with instructions or references to instructions on setting up cross section libraries for each supported depletion code.
• Updates should deploy to gh-pages automatically

Potential snags
Can't think of any.