Refactor the pages configuration

[d0ugal]

The current pages configuration looks like this:

pages:
- ['index.md', 'Home']
- ['writing-your-docs.md', 'User Guide', 'Writing your docs']
- ['styling-your-docs.md', 'User Guide', 'Styling your docs']
- ['configuration.md', 'User Guide', 'Configuration']
- ['about/license.md', 'About', 'License']
- ['about/release-notes.md', 'About', 'Release Notes']
- ['about/contributing.md', 'About', 'Contributing']

This has a number of flaws:

• It isn't clear how to add second levels of navigation for
  newcomers. This is often queried and not easy to document.

• We are representing a tree structure as a set of flat items
  that need to be merged. This creates some interesting edge
  cases, for example:

  • ['writing-your-docs.md', 'User Guide']
  • ['styling-your-docs.md', 'User Guide', 'Styling your docs']

  Is the first entry a page with the title User Guide? or a page
  in the User Guide category with an automatic title.

• We are currently limited to two levels deep in the navigation.
  Changing this with the current structure isn't trivial.

This change adds a new format which makes the above configuration
look like this:

pages:
- Home: index.md
- User Guide:
    - user-guide/writing-your-docs.md
    - user-guide/styling-your-docs.md
    - user-guide/configuration.md
- About:
    - License: about/license.md
    - about/release-notes.md
    - Contributing: about/contributing.md

(Note I added some tiles on purpose and skipped some to show usage).

With this structure, we can more easily see the documentation
tree and it is far more obvious what the navigation will look
like. It also removes the ambiguous edge cases and opens up the
possibility of adding further levels to the navigation more
easily.

This change restructures the pages configuration, but doesn't yet
allow users to add further levels in the navigation.

See #6

[ci5er]

I never personally had a problem with the old way, but this is pretty dang sweet. Thank you.

[waylan]

Wow, that is a great improvement. At a glance, the page structure is immediately obvious from the config. I like it.

[d0ugal]

This is now supported in the MkDocs theme.

[d0ugal]

[d0ugal]

Rebased to fix a conflict. This is working really well, I'm going to land it if the build is okay. For now just the mkdocs and ReadTheDocs themes will support >2 levels of nav.

[tobegit3hub]

Looking forward to this feature 😄