Add documentation project URL

[Herrtian]

Adds the current Read the Docs URL to the project metadata so the docs target is explicit.

Closes #88.

Checks:

• pre-commit run check-toml --files pyproject.toml
• pre-commit run validate-pyproject --files pyproject.toml
• uv build
• git diff --check

[henryiii]

Formatting of the PR description is bit off. :)

It would be nice to avoid a pep number in the readthedocs site, I think that's an open issue.

[Herrtian]

Thanks, fixed the body formatting. The scikit-build-core downstream failure looks unrelated to this metadata URL change; it is failing in its own Python 3.14/module import and setuptools-scm warning tests.

[henryiii]

I need to bump the version we use, probably. Looks like something I've fixed in the past.

[henryiii]

Not sure what's wrong with code coverage on this branch explicitly, but this change can't lower coverage. :)