This commit adds a postprocessor to the docs build that enables
examples to be edited and have the results show up live. See the
additions to `docs/README.md` for more about what writing these
blocks looks like or the netlify PR preview
(e.g. at `docs/edge/how-do-i-write-policies/`)
to see them in action (you'll need to disable CORS for live output).
Signed-off-by: David Boles <me@davidbol.es>