Update the quickstart `docker run` command after a stable release of datacommons-website-compose is cut

[wittrock]

This commit changed the required location of the sample data for custom_dc from /userdata to the value of $OUTPUT_DIR.

The stable release of datacommons-website-compose (9d47232847c9) was cut before this change, so it still requires the sample data to be mounted at /userdata.

I couldn't figure out a way to create a command that works for both the old sample data location and the new one, so instead I'm sending a PR to revert the docs command and add a TODO to update it again once there's a new stable version of the datacommons-website-compose image.