support versioned standards

[alee]

https://discourse.gohugo.io/t/documentation-site-versioning/5898/4

https://stackoverflow.com/questions/47643881/github-pages-maintaining-multiple-versions

https://www.docsy.dev/docs/adding-content/versioning/

Possible strategies:

1. All standards evolve together with a single semver or datever style version number (e.g., kubeflow / django / readthedocs etc
2. version standards independently as @cpritcha suggests in each standards subfolder with links to current | stable | dev
3. any others to consider?

Other considerations include embedding a citation.cff file with each versioned standard that properly credits the authors of the standard #441

[cpritcha]

The way to version mentioned in the docsy dev docs is https://www.docsy.dev/docs/adding-content/versioning/ (this is used for the version dropdown at https://www.kubeflow.org). To keep use from maintaining multiple versions however I think it would be easiest to have subfolders for in each standard folder (accessibility, documentation etc) for our old standard versions. Then we only have to maintain the one. I think we could still setup our site the kubeflow way on gh-pages by rendering old git tags of versions into a special folder but that would still force us to either have same version number across all our standards or split the repository into one standard one repository form.