Docs: Media creation vs. deploy/install instructions

[christophermay07]

From Install.md: "Directory Structure Assumptions" (section header), "Update Environment Variables" (subsequent section header)

Redline text (Directory Structure Assumptions): "This should be in two areas, one for Media Creation, second for Field Deployment/Install since they refer to very different parts of the process/timeline"

Redline text (Update Environment Variables): "This should be in two areas, one for Media Creation, second for Field Deployment/Install since they refer to very different parts of the process/timeline (Variables relevant to internet box, and variables relevant to syscon/bastion - There's also been discussion about should we potentially have two cluster-deployment.yml since some variables Only ever get accessed in one environment or another, and never both.)"

Update the docs to reflect instructions for both scenarios / personas. If it would be appropriate, split this into two issues, one for each section header.

[mriensch]

changed the flow to more inlined with this in riensch-working branch