[docs] Update sphinx configuration

[pandafy]

• Use sphinx_rtd_theme
• Add necessary packages in requirements-test.txt
• Reference: https://github.com/openwisp/netjsonconfig/blob/master/docs/source/conf.py

[purhan]

TODO: add versioned documentation (reference: https://github.com/openwisp/netengine/pull/54#discussion_r637816669)

[nemesifier]

Let's take advantage of this to also update authors and descriptions: https://github.com/openwisp/netengine/pull/59/files#r642714279