Allow sections to be configured in `nav`

In order to achieve top-level sections (e.g. to utilize material navigation tabs), I need to either:

• use this plugin but restructure all of my docs based on those sections
• use the native mkdocs nav functionality, and miss out on the good features here including the spread operator for the “rest” of the pages

It would be great to be able to specify custom sections in the .pages file of the docs_dir, e.g.:

nav:
  - Vision:
    - principles.md
    - roadmap.md
  - People:
    - team.md
    - advisors.md
  - ...

[lukasgeiter]

Good news! I did some quick tests and should be possible. Here’s how it’s going to work:

If a ... entry is present, the plugin generates a second navigation tree based off the file structure. All pages explicitly mentioned in nav are removed from this navigation tree before it is inserted in place of the ... entry.

It’s going to take me a bit of time to properly implement and test this. I’ll let you know once the release is out.

[athal7]

Works great, thank you @lukasgeiter!