Add Read the Docs build config

[ppolewicz]

Summary

• add a checked-in Read the Docs config for this repository
• install the system packages required by the Sphinx docs build on RTD
• install the Python doc dependency group with uv so RTD builds match local/CI docs builds

Why

The repo currently builds docs in CI, but CD does not publish docs and the repository does not include a .readthedocs.yaml file for RTD to consume. The docs build also requires graphviz and plantuml, which are installed in GitHub Actions but were not declared for RTD.

Verification

• uv run nox --non-interactive -vs doc

[jan-sauer-reef]

👍