Prevent old versions of the documentation from being indexed by Google

[hierophect]

A common issue when searching for Circuitpython's ReadTheDocs hosted documentation is that the pages for module APIs returned by Google are often very old, sometimes even going back to version 3.x or even 2.x. A prominent warning has been added to these pages to prevent users confusing old APIs for current ones, but a better solution would be to prevent these pages from being indexed by search engines altogether.

I found a Github issue for readthedocs started by the Astropy project that mirrors this exact issue. This conversation indicates a solution is possible, documented here: https://docs.readthedocs.io/en/latest/faq.html#how-can-i-avoid-search-results-having-a-deprecated-version-of-my-docs

An example of Astropy's implementation of this feature is in their PR here: astropy/astropy#8383

[hierophect]

@kattni @sommersoft pinging for your attention.