Add support for Sphinx tags in Read the Docs build process

[manivardhanreddythalla2007]

Summary

This PR adds support for Sphinx tags (.. only:: tagname) in Read the Docs builds.

Implementation

• Detects tags defined in conf.py
• Automatically adds them as -t tagname to the Sphinx command
• Ensures .. only:: directives render correctly

Example usage

# conf.py
tags.add('footag')

[stsewd]

We won't be adding this feature (please open an issue for feature requests). Also, not sure if it was done on purpose or not, but this basically adds a backdoor.