readme: update link system, words, & ex. semantics

- Links from `[title](url)` to `[title][short-name]`.

- Add links to dev.google.com docs next to the product page links (ex: [Google Cloud Pub/Sub][cloud.google.com page] ([docs][dev.google.com page])

- Use more direct wording in pubsub example to illustrate the pulling logic behind listener creation & removal

-