Dockerize *-docs Makefile commands

[igaskin]

Summary

Add an option to build the documentation for a docker container

Motivation

Running make build-docs calls out to mkdocs directly, which assumes that contributors have mkdocs installed locally and has dependencies on various python packages

Proposal

All *-docs make commands should be have options to execute in a dockerized environment. This brings this generation command in-line with how other code generations commands can operate.

[jessesuen]

Argo-rollouts takes this approach. We were able to use an off-the-shelf mkdocs image. See: https://github.com/argoproj/argo-rollouts/blob/master/Makefile#L155-L156

Here are Argo CD dependencies: https://github.com/argoproj/argo-cd/blob/c7c554674ec23c4c130ba5a6710e846a48ceb0c0/.github/workflows/gh-pages.yaml#L22

[ishitasequeira]

Looking into this issue