Add docstrings for nox sessions

[agriyakhetarpal]

Description

Adds descriptive docstrings for nox sessions, which can be listed with the nox --list or nox -l commands, and some general improvements. Closes #3048

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all
• The documentation builds: $ python run-tests.py --doctest

You can run unit and doctests together at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[agriyakhetarpal]

The docstrings are kept as single-line only because the first line will be printed when listing sessions. There is an open feature request to print the full docstring of a session (wntrblm/nox#694)

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (1998428) 99.71% compared to head (1e98fa5) 99.71%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3183   +/-   ##
========================================
  Coverage    99.71%   99.71%           
========================================
  Files          248      248           
  Lines        18761    18761           
========================================
  Hits         18707    18707           
  Misses          54       54           

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[agriyakhetarpal]

I also disabled colour output in the CI by default since we would not need it in non-interactive shells usually, and I refactored the session arguments (a bit) with nox.options

[Saransh-cpp]

Awesome, thanks, @agriyakhetarpal!