make search only consider stable/

[orbeckst]

Change the algolia search config so that

• docs.mdanalysis.org/stable is the only starting point for doc search
• dev, latest, MAJOR.MINOR.PATCH.* are stop urls and not indexed

In this way, search from the home page will only show the latest released features. Users can still use the doc's version selector to switch to different docs and use the sphinx search for each doc.

See #155 for discussion.

[orbeckst]

This has been confusing for users. We see users referring to outdated 1.1.1 docs.