Update documentation template

[Katetc]

Our documentation uses outdated code to allow for versioning. A recent change to the CIME documentation in PR#4613 changes the way the version drop-down works in the documentation to something that allows for modern python builds and better code. We should update our documentation to include this as well.

[whlipscomb]

@Katetc, I'd be interested to follow up on the details in person. I'm in favor of anything that helps us maintain good documentation.