Split up campaigns docs into getting started, how-tos, tutorials

Created by: mrnugget

Disclaimer:

• CSS is probably horrible
• I won't redo all of it in this PR (i.e. yes, the "introduction" section needs more work, but I'll take that in another PR)

Preview

docs.sourcegraph.com/@mrnugget-campaign-doc-reorg/user/campaigns

Before

After