docs: improve high-level structure for the documentation

[lalithsuresh]

Signed-off-by: Lalith Suresh suresh.lalith@gmail.com

I'm trying to reorganize the top-level structure. Unfortunately the current content makes it hard to change a few key things:

• Demo/tour is somewhat redundant with the tutorial. They needn't go into separate sections.
• The Demo/tour is locked to working only with the docker install, which means the docker installation instructions should precede it in the "Get started" section.
• Ideally, the deployment form factors should show up one top-level section. The rest of the documentation (demos, tutorials etc) should have a context button that switches instructions depending on whether the user is running against the Docker or Kubernetes FFs.
• The Reference section could use some groupings (e.g., all formats in one) and a better title.
• The Learn section could use some love. It's pretty dated.

Current structure with this PR:

Expanded:

Is this a user-visible change (yes/no): yes

[blp]

Seems like a good start to me.

I always thought that the demo, tour, and tutorials were redundant. We should figure that out.