Prevent search engines from indexing documentation of old package versions

[eonu]

Currently when you search for Sequentia documentation on search engines, the documentation for some of the older package versions is the first to appear. A good example of this is if you search 'dtw knn' on Google, the documentation for the old DTWKNN class from v0.6.1 appears.

This can obviously be confusing for users, so it would be good if we could possibly add a docs/robots.txt file to control what pages are indexed.

astropy/astropy#7874 (and their robots.txt) is a good example of adding this functionality.

Another option is to simply add a warning banner to the top of any documentation page which isn't for the latest package version (example). The sphinx-version-warning Sphinx extension seems to achieve this.

[eonu]

Addressed in #147.