Use git submodule to import osm docs into this repo

[michelleN]

We want Azure customers to have one place to see all osm related documentation whether that may be azure/arc specific or oss specific. Jumping around repos to figure things out is a pain. It'd be great if we could import the oss docs into this repo perhaps using a git submodule. git submodules also allow us to pin to a specific version of osm so folks don't need to go to the main osm repo and go through another step of checking out the relevant git tag. This is an exploratory issue. I'm not sure if you can just import one directory (docs/) with git submodules so that is one particular thing we'll need to explore too. As part of this issue, we'll also want to note what the experience/process/workflow is for osm-azure maintainers to actually maintain the git submodule in the long term.