polish up docstrings, etc. in skater.py

[jGaboardi]

This PR:

• polish up docstrings in region/skater.py
• converts formatted string to f-strings in region/skater.py
• makes a couple formatting correction in region/base.py
• cleans up references in docs/_static/references.bib
• change a raw Exception to a ValueError in region/skater.py

xref #173, #280, #331

[codecov]

Codecov Report

Merging #335 (179607d) into main (f2f54ef) will not change coverage.
The diff coverage is 88.9%.

@@          Coverage Diff          @@
##            main    #335   +/-   ##
=====================================
  Coverage   74.0%   74.0%           
=====================================
  Files         23      23           
  Lines       2477    2477           
  Branches     513     513           
=====================================
  Hits        1833    1833           
  Misses       580     580           
  Partials      64      64           

Impacted Files	Coverage Δ
spopt/region/base.py	97.6% <ø> (ø)
spopt/region/skater.py	82.7% <88.9%> (ø)

[jGaboardi]

@knaaptime Thanks for the review!