Add docstrings for more methods

[abhro]

• Add docstrings for ΛCDM types, a2E, scale_factor, E, H, hubble_dist, hubble_time (and their present counterparts), Z
• Add project dependency on DocStringExtensions
• Add docs dependency on DocumenterCitations
  • Add citations for some of the code used
• Add new page for internals, so that it can serve as an easy reference for package devs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (2cb1082) to head (6bc463e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #51   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           74        74           
=========================================
  Hits            74        74           

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[abhro]

The doc previews keep failing, requesting someone with permissions to look at Documenter keys