Add a [source] to docs [feature request]

[lockwo]

For users that are interested/regularly investigate the source code, having a method to go from docs to associated source code is very helpful (this is what I am talking about: https://jax.readthedocs.io/en/latest/_autosummary/jax.nn.log_sigmoid.html there is a [source] link that when clicked takes you to the actual source code). I find myself going to google, which takes me to the docs of a method, then having to go through the repo to find it so I was wondering if this an interesting/possible feature for the docs?