Clean up and extend the campaigns how-tos in the docs

Created by: mrnugget

I want to improve our how-tos before we launch:

• All of them need proper subheadings/sections
• Creating a campaign needs
  • Screenshots
  • Links to requirements (like the quickstart guide)
  • Explanation of "preview" vs. "apply"
• Updating a campaign needs
  • Explanation of which changesets are detached and closed
  • Screenshots
  • Rename "Update campaign" to "Apply spec"
• Publishing changesets to the code host
  • Subsection for publishing subset
  • Subsection for publishing-as-draft
  • Subsection as draft-to-non-draft
  • Fix indentation of header
  • Mention of force push! (Or should this go in an Explanation?)
  • Mention of "reapplying will override the commits" (Or should this go in an Explanation?)
• Closing or deleting a campaign
  • Screenshots
  • Fix indentation of header
• Viewing campaigns - Sourcegraph docs
  • Add section about filtering changesets
  • Add screenshot
• Tracking existing changesets - Sourcegraph docs
  • Screenshots