Restore table of contents on documentation landing pages

[ryanlovett]

Summary

The old .rst site had landing pages for major sections containing a sub table of contents for those sections. Quarto does not support this natively, but it may be possible to move the main TOC from _quarto.yml to a separate YAML file, and then reference that in _quarto.yml and in an EJS template that populates the sub TOCs.

I imagine a landing page could have:

title: "Some Landing Page"
listing:
  - id: toc
    template: assets/toc.ejs
    contents: _toc.yml

## Table of Contents

This section's pages:

::: {#toc}
:::

I'm not sure how to reference a section within the main _toc.yml from this listing and template, but I haven't looked into it.