Suppress 'rtd-footer-container' if not being built on ReadTheDocs

[DocOtak]

This PR attempts to fix #1794 and possibly #1238 by not including the rtd-footer-container div if the site is not being built on ReadTheDocs.

This uses the same {% if READTHEDOCS %} as the ethical ads html.

[DocOtak]

Two alternatives to this I think are either to give the rtd-footer-container a min-height of 1rem. Or set the bottom margin to 0. The negative top and bottom combined with the zero height of this div when there is no content has the effect of pulling the div 1rem bellow its parent's bottom.

edit: I think doing the min-height option might be more correct for what I think is intended