privacy: GoatCounter data are stored for half a year

- updated policy
- updated retention time on https://mdanalysis.goatcounter.com/settings
- close #138

pages/privacy.md

@@ -51,6 +51,7 @@ We only share information with the following third parties:
 * Usage statistics: [GoatCounter][] collects anonymous page visit
   statistics, as explained in
   [https://www.goatcounter.com/privacy](https://www.goatcounter.com/privacy).
+  Data are stored for up to 182 days on GoatCounter's servers.
   These statistics are publicly visible at
   [https://mdanalysis.goatcounter.com](https://mdanalysis.goatcounter.com/).
 * Website hosting: [GitHub](https://github.com/) hosts the web pages