Fix docs site CD and clean up old system

[becky-gilbert]

Summary

Our automated docs build system is not working. We have a GH webhook set up to trigger a RTD build, but GH is showing an error ("Last delivery was not successful. Invalid HTTP response: 400") and the RTD integrations log is showing that GH has been sending build triggers to RTD but they have not been successful. The RTD build is successful when triggered manually.

For info:
GH: Settings -> Webhooks -> Select the one for https://readthedocs.org/
RTD: ember-lookit-frameplayer project -> Settings -> Integrations -> Github incoming webhook

I think this repository also includes some carry-overs from the old EFP documentation site's CD process that are now obsolete. This is confusing and has made it harder to debug issues related to CD on our current EFP docs site.

To do

Figure out why the GH -> RTD build trigger isn't working.

And assuming these things that are related to the old docs site are no longer used or needed, we should do the following:

• remove the old 'docs' directory from the project root
• delete/disable the GH action for building/deploying docs to the old site
• delete/update the out-of-date documentation about EFP documentation deployment, e.g. here and in our internal docs