[MRG] Bump website theme

[drammock]

closes #143

PR Description

• updates to use the latest patch release of the website theme
• remove some crufty templates
• switch to using JSON file for the version switcher dropdown (instead of html_context)

Merge checklist

Maintainer, please confirm the following before merging:

• [ ] All comments resolved
• [ ] This is not your own PR
• [x] All CIs are happy
• [x] PR title starts with [MRG]
• [x] whats_new.rst is updated (not really necessary)
• [x] PR description includes phrase "closes <#issue-number>"

[drammock]

version switcher works in the rendered CI build:

[drammock]

this is good to go as far as I'm concerned.