add GoatCounter tracking to docs

Add

the GoatCounter tracking snippet
```
<script data-goatcounter="https://mdanalysis.goatcounter.com/count"  
       async src="//gc.zgo.at/count.js"></script>
```
just before the </body> tag (better for performance) or anywhere in <head> in all generated pages.
Link to the privacy policy https://github.com/MDAnalysis/MDAnalysis.github.io/issues/129

to the generated API docs.

MDAnalysis / pmda