Updates to the 'Launch a stack' section

Updated the following:
- Included new procedure to 'Manage a stack'.
- New steps to select the template and environment sources for the stack.
Partial-Bug: #1307860

Change-Id: I5fab0587ce7a81e5092f9e089b4cbb2197c7b8d0
Merge "Updates to the 'Launch a stack' section"