Have a way of linking to the current version of a documentation page in the topnav bar #308

This was originally brought up here: astropy/astropy-sphinx-theme#48 (comment)

Currently we have the following situation:

• Navigate to docs.sunpy.org/en/latest
• Go to the Documentation > sunpy option in the top navbar
• You are now at docs.sunpy.org/en/stable

It must be possible to have the sunpy docs sphinx build know what the root of the current documentation page is and have it substitute the link in the navbar, I just don't know how.