Move example code in pkg/example to examples/

[chungers]

This PR moves the example binaries, such as instance-file, instance-terraform and flavor-swarm to a top-level directory instead of pkg/example where they currently are:

• This makes the examples easier to find.
• The directory pkg/ is reserved for code in InfraKit that are meant to be imported by other projects. The main packages in the examples/ directory show how this is done.
• Moved the e2e test / demo script for Docker Swarm flavor to a better home in examples/flavor/swarm
• Updated docs and references; made sure there are no dead links.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "move-example" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353744704
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov-io]

Current coverage is 64.82% (diff: 100%)

Merging #365 into master will decrease coverage by 0.13%

@@             master       #365   diff @@
==========================================
  Files            47         47          
  Lines          2266       2266          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits           1472       1469     -3   
- Misses          636        637     +1   
- Partials        158        160     +2   

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