Update readthedocs page

[fehiepsi]

@fritzo There are a few black changes remaining, hope that they are fine. I have incorporated (and tested) the following changes that I made to rtd page:

• add the ability to add examples/tutorials to index.rst file
• add the ability to generate thumbnail galleries (this is not necessary)
• add colab links to rendered tutorials
• each tutorial/example has a link that points to the source file (with correct version) in github.

I need to add __version__ to funsor.__init__.py to generate correct links for local versions (not sure if it is needed for funsor.pyro.ai). We expose __version__ in pyro and numpyro so it is fine to do it here I guess.

[fritzo]

Thanks @fehiepsi, this is great!