Rework the EXAMPLES section to be a bit clearer.

• Add an example of using etcupdate diff.
• Create a subsection on bootstrapping that is below the simple examples. This should make it clearer that 'etcupdate extract' is a one-time operation and not part of the common workflow. It also adds more suggestions on when bootstrapping is needed and additional steps to make future merges simpler.

Reviewed by: adrian
MFC after: 3 days