feat: converting documentation to mkdocs

[gvwilson]

Paired with plotly/plotly.js#7469 (formatting changes to JavaScript that Python docs are derived from).

• Re-introduce links between .md files in root directory (taken out because they don't resolve when those files are transcluded from pages/*.md).
• Include content from docstrings in hand-written Python code.
• Include content from docstrings in generated Python code.
• Include navigation links in left nav bar.
• Clean up warnings produced by mkdocs (most resulting from poor formatting of source material).
• Add some kind of linting for docstring style (currently supposed to be google according to mkdocs.yml).
• Move scripts/gen_ref_pages.py into commands.py. (Can't do this because mkdocs.yml only accepts the name of the script, not parameters.)