Since all built-in block Sass is gone, we remove two final pieces of code: * the steps in `npm run compile-sass` which compiled `xmodule/assets`, and * the now-unused `add_sass_to_fragment` function. This should speed up the edx-platform assets build a little bit. This commit also includes some minor dev doc updates. Part of: https://github.com/openedx/edx-platform/issues/35300