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.
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