refactor auto-nav to use {{< insert >}}

[andrewpbray]

Instead of having users specify where they want auto-navigation via the auto-nav key, allow the use of a shortcode like {{< insert Notes >}} anywhere within the existing description of the website nav in _quarto.yml.

Simplified example

If the aim is for the project scripts to produce an adaptive sidebar nav that pulls from all items where type: Notes in course-website: schedule to produce, say:

website:
  sidebar:
    logo: assets/stat_bear.png
    contents:
      - href: index.qmd
        text: Home
      - href: syllabus.qmd
        text: Syllabus
      - section: Notes
        contents:
          - notes1.qmd
          - notes2.qmd

Here are two ways that the user could specify this.

Existing version

website:
  sidebar:
    logo: assets/stat_bear.png
    contents:
      - href: index.qmd
        text: Home
      - href: syllabus.qmd
        text: Syllabus

course-website:
  auto-nav:
    sidebar:
      - type: Notes

Proposed version

website:
  sidebar:
    logo: assets/stat_bear.png
    contents:
      - href: index.qmd
        text: Home
      - href: syllabus.qmd
        text: Syllabus
      - section: Notes
        contents:
          - "{{< insert Notes >}}"

Rationale for the change

1. More flexibility in the structure of navigation. The user hard-codes the structure that they want into website: sidebar and just uses the shortcode as a variable where the adaptive content will be inserted. In the existing version, we assume a simple nav structure when we write the contents yaml.
2. No need to match structure/syntax. In the current setup, the user has to learn how to use the options under auto-nav to be sure they match where they want the contents yaml to show up in the nav. This eliminates the need for auto-nav entirely; they can just read the quarto docs for understanding website nav.