docs/source/conf.py: Use the read the docs theme if available

It is useful to build avocado docs locally and have them
look mostly identical to the reference documentation,
hosted in read the docs. The theme can be installed using
pip with:

pip install sphinx_rtd_theme

And if not present, the build should gracefully
degrade to the sphinx default.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
README.rst: Add references to the online docs

Also, add instructions to build the docs for offline
viewing.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #26 from avocado-framework/sphinx-theme

HTML documentation theme and README.rst improvements