document how Tutor installs `sample-plugin` from NPM

[brian-smith-tcril]

I couldn't find anything in https://github.com/openedx/sample-plugin/tree/main/frontend or https://github.com/openedx/sample-plugin/tree/main/tutor that mentions this step. This part is documented in the Tutor MFE README.

From https://github.com/overhangio/tutor-mfe?tab=readme-ov-file#using-frontend-plugin-slots:

hooks.Filters.ENV_PATCHES.add_item(
    (
        "mfe-dockerfile-post-npm-install",
        """
# npm package
RUN npm install react-loader-spinner
""",
    )
)

hooks.Filters.ENV_PATCHES.add_item(
    (
        "mfe-env-config-buildtime-imports",
        """
import { FidgetSpinner } from 'react-loader-spinner';
""",
    )
)