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