migrate docs from Sphinx/RST to MkDocs Material

[jf---]

Summary

• replace Sphinx with MkDocs + Material theme
• use mkdocstrings for API docs from NumPy docstrings
• convert 73 RST files to Markdown
• remove RST directives from Python docstrings
• update CI workflow for mkdocs gh-deploy
• update invoke tasks for mkdocs commands

Test plan

• mkdocs serve works locally
• mkdocs build --strict passes
• API docs render correctly
• all navigation links work