Update writing addons documentation

[m10l]

This PR achieves two things - updates an outdated dependency in a code example, and inlines all code examples rather than relying on external Gists.

[codecov]

Codecov Report

Merging #2951 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2951   +/-   ##
=======================================
  Coverage   37.37%   37.37%           
=======================================
  Files         426      426           
  Lines        9151     9151           
  Branches      877      871    -6     
=======================================
  Hits         3420     3420           
+ Misses       5190     5180   -10     
- Partials      541      551   +10

Impacted Files	Coverage Δ
app/react/src/server/utils.js	0% <0%> (-100%)	⬇️
app/polymer/src/server/babel_config.js	0% <0%> (-76.67%)	⬇️
addons/actions/src/lib/util/getPropertiesList.js	60% <0%> (ø)	⬆️
addons/a11y/src/components/Report/RerunButton.js	0% <0%> (ø)	⬆️
addons/actions/src/lib/types/symbol/index.js	81.25% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/layout.js	12.5% <0%> (ø)	⬆️
lib/core/src/server/build-dev.js	0% <0%> (ø)	⬆️
addons/info/src/components/types/PrettyPropType.js	86% <0%> (ø)	⬆️
addons/actions/src/lib/reviver.js	58.33% <0%> (ø)	⬆️
addons/storyshots/src/angular/renderTree.js	61.9% <0%> (ø)	⬆️
... and 50 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4cf44c0...8eecd23. Read the comment docs.