Document the docs site implementation

For someone new to the tooling involved in making https://docs.gitlab.com happen, it would be good to have documented:

• The content here: https://about.gitlab.com/2016/12/07/building-a-new-gitlab-docs-site-with-nanoc-gitlab-ci-and-gitlab-pages/, but in a permanent and sectioned way.
• Taking some content in from the Getting Started with Docs at GitLab hackathon presentation.

Steps:

• Start at a high level in gitlab-docs wiki. Include:
  • Components to know about: the repo, nanoc, /help, etc.
  • Steps (simple numbered lists?) of how docs are built; maybe multiple processes in parallel
  • Record a video with details for any part of the process (lower effort but useful)
  • Commit details from video to the doc