add privacy notice and GoatCounter to RTD theme

- updates #79
- see also MDAnalysis/MDAnalysis.github.io#129 (Privacy Policy)

Author: orbeckst

doc/source/_templates/layout.html doc/source/_templates/footer.htmldoc/source/_templates/layout.html renamed to doc/source/_templates/footer.html

@@ -1,6 +1,6 @@
-{% extends "!layout.html" %}
+{% extends "!footer.html" %}
 
-{% block footer %}
+{% block extrafooter %}
 {{ super() }}
 <div class="footer"><p>Please see
     our <a href="https://www.mdanalysis.org/pages/privacy/">Privacy Policy</a>