Remove _build and _static directories

[scottyhq]

Since GitHub Actions are now configured to build and publish the rendered website automatically, the build folders can be removed from the main branch.

Note just removing the folders they will continue to take up disk space when the repository is cloned. Fully removing these files from the repository history is more involved but also could be done - see https://docs.github.com/en/repositories/working-with-files/managing-large-files/about-large-files-on-github#removing-a-file-that-was-added-in-an-earlier-commit

296M	_build
4.0K	_config.yml
1.2M	_static