docs: add pydocsstyle linting and switch to mkdocs

[kenibrewer]

Description

I was working on #382 and couldn't figure out how to resolve some sphinx build errors with opaque error messages. I decided to take this opportunity to switch our docs engine to mkdocs instead.

• Enables pydocstrings linting checks
• Various documentation strings changes to conform with linting checks.
• Switch from Sphinx to Mkdocs
• Added docs build ci check

What is the nature of your change?

• [x] Bug fix (fixes an issue).
• [x] Enhancement (adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [x] This change requires a documentation update.

Checklist

Please ensure that all boxes are checked before indicating that a pull request is ready for review.

• [x] I have read the CONTRIBUTING.md guidelines.
• [x] My code follows the style guidelines of this project.
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.
• [x] New and existing unit tests pass locally with my changes.
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have deleted all non-relevant text in this pull request template.

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📚 Documentation preview 📚: https://pycytominer--396.org.readthedocs.build/en/396/