Add documentation on spack modules

[pearce8]

Where should this live?

• A sub-bullet under "Adding a Specific System configuration"
• FAQ: I would like to use external dependencies ... Adding a Specific System could point here.

Explain that if installed software does not have correct rpaths (e.g., it was installed by EasyBuild), it needs to be added as a spack external module, not just a spack external.

Point at Spack modules documentation.

[becker33]

https://spack.readthedocs.io/en/latest/packages_yaml.html#external-packages is the correct Spack documentation ref. If you search modules in the spack docs you'll get all results about Spack-generated modules which is a different issue.