📝 Document need for container platform software

[shnizzedy]

Related problem

The quickstart guide and cpac page in the documentation don't explicitly state that a container platform needs to be installed (and in the case of Docker, that the daemon needs to be running) for cpac to work.

Proposed feature

We should clarify that that a user needs at least one of these installed (and in the case of Docker running) for cpac to work.

Acceptance criteria

• Quickstart guide links to Docker and Singularity installation instructions
• cpac page links to Docker and Singularity installation instructions

Additional context

From my perspective these are the main pros and cons:

	Pros	Cons
Docker	• more mature • more documentation • wider feature set	• larger footprint on disk • requires special permissions • requires daemon to be running
Singularity	• lighter weight • no special permissions required • only runs when called	• less mature • less documentation • narrower feature set