IMPORTANT Add a robots.txt to stop Google from indexing Deprecated Versions of the Docs

[aribornstein]

Google is currently indexing deprecated versions of the docs so that when you search for features you are getting depricated information. For example if I google "pytorch lightning save best model" The first result is associated with lighting version 0.4.9 with no indictation that isn't the latest version of the docs.

This shouldn't be that hard to fix and will have major impact on developer experience.
https://docs.readthedocs.io/en/latest/faq.html#how-can-i-avoid-search-results-having-a-deprecated-version-of-my-docs

[aribornstein]

#6488

[akihironitta]

#6160