Add etstool task for local documentation viewing?

Previously I've been testing documentation builds by building locally with `sphinx-build` and then simply doing `open docs/build/index.html`. But that now runs into browser security precautions: search fails to work properly because it refuses to load content from local files. Instead, the right way to view the documentation is to set up a local HTTP server (which the Python standard library makes a trivial task: just navigate to the directory containing the top-level `index.html` and do `python -m http.server`).

It may be useful to have an `etstool.py` docview task for this.