Update the version displayed in the header on RTD to 2.0.0

Two out of three ain't bad.

<img width="816" height="172" alt="Image" src="https://github.com/user-attachments/assets/2db5ee67-2aae-4ede-95cf-769ed2d02718" />

Although a simple fix by changing the version number in `conf.py` is a quick fix, I'd prefer it to read the version from `pyproject.toml`. It's best to set it in one place and update it everywhere.