Document docstring requirements with examples

[tpdorsey]

Updates contribution guide with details on docstring requirements.

Change Summary

• Reorganize "Documentation style" contribution guidelines with details on docstring style and linting.
• Provide examples of correct docstrings
• Add ignore_init_summary: true and merge_init_into_class: true configuration. This will enable correct documentation of class vs instance attributes -- instance attributes should be documented in __init__, and mkdocstrings will pull this into the class documentation. See https://mkdocstrings.github.io/python/usage/configuration/docstrings/#merge_init_into_class for background.

The merge_init_into_class: true configuration change does not affect API documentation currently, but enables documenting instance attributes correctly in future PRs.

Related issue number

Closes DOC-4
Closes DOC-5

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[linear]

DOC-5 Establish and document internal module docstring requirements

And add them to the contribution guidelines

DOC-4 Establish and document public module docstring requirements

And add them to the contribution guidelines

[tpdorsey]

Please review