Migrate docs to Sphinx and Read the Docs

[MaxGhenis]

Summary

• replace Jupyter Book config with Sphinx + MyST config
• add Read the Docs build config
• turn the GitHub docs workflow into PR/main build verification with warnings as errors
• update docs dependencies and lockfile to remove the Jupyter Book notebook stack

Verification

• UV_FROZEN=0 uv run --extra docs sphinx-build -W --keep-going -b html docs docs/_build/html
• UV_FROZEN=0 uv run --python 3.11 --extra docs sphinx-build -W --keep-going -b html docs docs/_build/html-py311
• UV_FROZEN=0 uv run --extra dev ruff check docs/conf.py
• git diff --check

Deployment note

The code is ready for Read the Docs, but the microplex project is not imported there yet. Import CosilicoAI/microplex on Read the Docs before merging or immediately after merging so README/package documentation links resolve at https://microplex.readthedocs.io/.