doc: what is a publishing service?

[nelsonpecora]

We should have documentation about the idea of a "publishing service", and generally the workflow for generating urls and publishing

[jonwinton]

Changes to pub service in this PR: #455

[jonwinton]

Initial pass: http://clay.github.io/amphora/docs/basics/publishing.html

Assuming it will be supported by Clay narrative docs later. Should be enough for people to get up and running with the API, but new issues/PRs can be opened regarding the contents