Make Campaigns teaser pages clearer and easier to follow

Created by: mrnugget

This removes a lot of redundant information on the teaser pages (previously we had multiple mentions of "only available to site admins", multiple "reach out to us", ...) and makes the getting-started steps easier to follow.

Instead of having 2 call-to-action buttons and a list of steps that all link to the same documentation page, I changed it to a single list of steps that leads to the campaigns documentation's "Getting started" guide.

Until we can make bigger improvements to the UI that allows us to incorporate the "Getting started" guide into the product itself, I think it's better to not have redundant guides and provide the user with a single list of instructions they can follow step-by-step.

Screenshots

BEFORE: /campaigns on Sourcegraph instance

BEFORE: /campaigns on Sourcegraph.com

AFTER: /campaigns on Sourcegraph instance

AFTER: /campaigns on Sourcegraph.com