Fix mkdocs site_url to path-based docs URL

Docs are served at docs.cryptuon.com/<project>/, not at the
<project>.docs subdomain. Sitemap and canonical tags now match.

Author: dipankar

documentation/mkdocs.yml

@@ -1,5 +1,5 @@
 site_name: PolyBot
-site_url: https://polybot.docs.cryptuon.com/
+site_url: https://docs.cryptuon.com/polybot/
 site_description: "Open-source automated trading system for prediction markets"
 site_author: "Dipankar Sarkar"
 repo_url: https://github.com/cryptuon/polybot