update docs for new campaign flow

Created by: sqs

I am pre-writing the docs so we have a concise description of the new flow from the user's POV.

Preview as rendered docs at:

• https://docs.sourcegraph.com/@campaigns-new-flow/user/campaigns
• https://docs.sourcegraph.com/@campaigns-new-flow/user/campaigns/hello_world_campaign
• https://docs.sourcegraph.com/@campaigns-new-flow/dev/campaigns_design

Figma mock (slightly behind the docs pages above): https://www.figma.com/file/kRf5i7xf1p4zs3OyX0DUVp/Campaigns

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Docs that led to this change: RFC 157 and Campaigns CLI and naming/concepts idea. The proposals and motivations from those docs will be fully incorporated into this PR's .md doc changes.