Update Testing section in Makefile

[rhmdnd]

This commit applies some conventions to the targets and documentation in the testing section:

• Use single spaces before help text (results in shorter lines)
• Clarify build and push image targets for content
• Add documentatin for prep-e2e
• Add e2e target to the testing section
• Fix the must-gather target by adding push-must-gather-image

Fixes #159