docs: Integrate with readthedocs.org

This patch adds configuration to integrate with readthedocs.org, using
mkdocs as rendering engine.

It also does minor documentation updates, to tidy some things up
(clearer titles, move some documentation from the root into docs/, etc).

docs/monitoring.md

@@ -31,7 +31,7 @@ endpoint for expvars).
 
 *Note these are still subject to change, although breaking changes will be
 avoided whenever possible, and will be noted in the [upgrading
-notes](../UPGRADING.md).*
+notes](upgrading.md).*
 
 List of exported variables: