make version selector better discoverable

[ThomasWaldmann]

Suggestion from a user who overlooked the version selector:
"""
I just now saw the drop-down in the bottom right corner to select the different versions on that docs page. I wonder if that feature would be more intuitive/obvious if it were in the upper left corner of the page (like next to the logo/version).
"""

[DougInAMug]

The version selector is on the docs page, which seems to be a readthedocs instance with source files stored elsewhere... not sure how this issue can be solved from here.

[ThomasWaldmann]

@DougInAMug Hmm, right, the stuff here (for borgbackup.github.io) is separate from the versioned software docs auto-generated from borgbackup repo (for borgbackup.readthedocs.org).

I'll move this to the borgbackup issue tracker.