Add readthedocs/mkdocs integration to the documentation

[jgwest]

We can use the work done on https://github.com/argoproj-labs/argocd-image-updater/ as a template:

• Create a readthedocs account, with a name like argo-cd-application-set
• Add .readthedocs.yml, mkdocs.yml, and additional metadata from /docs like requirements.txt, plus css, js
• Import that into read the docs and iterate on it until it looks good :)

I have a prototype branch which seems to be working, so this should be straightforward.