Set priority attribute based on version

[jdillard]

Set <priority> as:

• 1 for the pages of the latest or stable version.
• for each following version, decrease the priority of 0.1 at each version
• 0.1 for the pages for other version if there is more than 9 versions.

The priority could possibly be a config value, sitemap_priority, manually set/configured in conf.py with the ability to be changed based on the versioning method (change the value for each tag/branch).

Taken from here: https://github.com/rtfd/readthedocs.org/issues/557