add _static/ hierarchy since it's mentioned in the conf.py
use sphinx_rtd_theme locally if availble

* do not set it up if the build happens at readthedocs.org
  (https://github.com/snide/sphinx_rtd_theme#using-this-theme-locally-then-building-on-read-the-docs)
* do not set it up if not available locally
Merge pull request #307 from sa2ajj/fix-doc-infra

Fix doc infra