Add link for GMT6.6

Author: Esteban82

documentation/index.rst

@@ -8,6 +8,7 @@ Documentation for GMT is available online in different versions (if in doubt, us
 
 * `Latest stable release <https://docs.generic-mapping-tools.org/latest>`__
 * `Current development version (unstable nightly builds) <https://docs.generic-mapping-tools.org/dev/>`__
+* `GMT 6.6.0 <https://docs.generic-mapping-tools.org/6.6/>`__
 * `GMT 6.5.0 <https://docs.generic-mapping-tools.org/6.5/>`__
 * `GMT 6.4.0 <https://docs.generic-mapping-tools.org/6.4/>`__
 * `GMT 6.3.0 <https://docs.generic-mapping-tools.org/6.3/>`__