Add summary section as Repositories main page

[pedro-psb]

To improve the navigability of the website, I want to add landing page for each repository listed with a generated ToC.

This should enable skimming through the entire summary, instead of having to expand all the sections one-by-one.

Requirements

Given the main left navbar, which list the repository namespaces (e.g "Pulp Core", "Pulp RPM", "Pulp CLI", etc)

As a user who want to find info about a specific Repository, when clicking on a Repository namespace I wanna see:

• The Repository section expanded + The Repository main page, which should contain:
  • An optional content provided by the Repository at {repository}/docs/index.md.
  • An auto-generated section (appended in the index.md, if it exists) with the entire ToC of that Repository.

[pedro-psb]

Some notes from early experiments:

# on hooks.py
def on_nav(nav, config, files):
    """
    Create repo-index-toc data here.
    See hook events: https://www.mkdocs.org/dev-guide/plugins/#events
    """
    # get each repository subtree in nav.items (Tree of Section/Pages)
    # for each subtree, serialize to markdown list structure
    # save serialized markdown string
    # write to files here or on_page events
    ...