Update documentation for RAVEN tasks

Over the past few years, the documentation in the RAVEN-related task modules have either become somewhat outdated or seem inadequate in comparison to other modules. The modules needed to be updated would be:

• external_triggers.py
• raven.py
• external_skymaps.py
• first2years_external.py (once merged in !845 (merged))

The goal is to bring these modules to the standard of superevents.py and to update the flow charts there to the current workflow (i.e. the actual code). See the following figure for a (near) comprehensive example of the workflow: https://git.ligo.org/brandon.piotrzkowski/raven-paper/-/blob/master/figures/Raven_diagram.pdf