Switch docs to mkdocs

[aliev]

Summary

• migrate Sphinx docs to mkdocs
• update Makefile to build and serve mkdocs
• update docs publishing workflow
• change docs extras in pyproject.toml
• remove old Sphinx files

Testing

• pip install -e ."[docs]"
• mkdocs build
• pip install -e ."[dev]"
• pytest -q

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https://chatgpt.com/codex/tasks/task_e_6885d0374a6c832c991a88c75830a17c