Update documentation heading

[rgaiacs]

BinderHub	JupyterHub
pydata-sphinx-theme	jupyterhub-sphinx-theme

In the navigation bar

• Change GitHub logo to round one
• Remove Twitter logo
• Add Discourse logo with link to https://discourse.jupyter.org/c/binder/binderhub
• Add Compass logo with link to https://jupyterhub-team-compass.readthedocs.io/en/latest/

In index page

• Replace badge "chat on Gitter" with "chat on Zulip"
• Remove badge "help forum"

I believe this changes are due the Sphinx theme. Should the theme be change to jupyterhub-sphinx-theme?

[manics]

I've got no objections to switching to jupyterhub-sphinx-theme. If you open a PR we can use the readthedocs automatical PR preview to check whether everything is OK.