NOTE: I believe this task will help implement T243281 more effortlessly.
From what I can see with the new version of Vector the easiest path to maintenance would be to maintain 2 master templates or to maintain a single template with conditionals related to version number.
In both scenarios we would benefit from the use of template partials to make index.mustache more readable and easier to rearrange.
= Acceptance criteria
 There is a single call to processTemplate inside VectorTemplate.
 Any new components are documented in storybook and have a template associated.
 Any revised components are updated in storybook
 Consider updating VectorTemplateTest or adding tests elsewhere
= Developer notes
Associated patches show an incremental way to do this.
For context on how this helps T243281 read T243281#5871879 and then consult [[ https://gerrit.wikimedia.org/r/#/c/mediawiki/skins/Vector/+/572740 POC: Opt in button | POC Vector patch ]]