CI: skip non-doc checks for docs-only changes

Created by: chrismwendt

This runs only prettier-check and docsite check on docs-only PRs. All steps still run on master.

See:

• https://github.com/sourcegraph/sourcegraph/issues/2706
• https://sourcegraph.slack.com/archives/C07KZF47K/p1552422899189900
• https://sourcegraph.slack.com/archives/C08JA8Q1H/p1552448146079500