Fix the way the sidebar renders top-level category index pages

[ptgott]

There are two problems with how the docs sidebar renders top-level docs category pages, e.g., "Admin Guides" and "User Guides":

• Top-level category index pages appear twice in the sidebar, once as an expandable section and once as a page within that section, and not necessarily the top page.
• Unlike sidebar categories at or after the second level, top-level categories do not link to their index pages when a user clicks on them.

For example, the section of the sidebar for "Admin Guides" includes the "Teleport Admin Guides" page as a page within the category, when it should be the page that readers visit when they click "Admin Guides".

Fix the docs site configuration so that clicking top-level categories on the sidebar navigates to the relevant index page.

[ptgott]

The sidebar generator is in node_modules/@docusaurus/plugin-content-docs/lib/sidebars/generator.js. What's strange is that, if I add the following console log to the end of the file...

     const docs = getAutogenDocs();
     const fsModel = treeify(docs);
+    console.log("fsModel", JSON.stringify(fsModel, undefined, 2));
     const sidebarWithPosition = generateSidebar(fsModel);
     const sortedSidebar = sortItems(sidebarWithPosition);
     return sortedSidebar;

The fsModel does contain the intended category index pages, but does not assign them entries. You can see this with reference.mdx in the output snippet below:

fsModel {
  "backends.mdx": "reference/backends",
  "cloud-faq.mdx": "reference/cloud-faq",
  "config.mdx": "reference/config",
  "join-methods.mdx": "reference/join-methods",
  "networking.mdx": "reference/networking",
  "predicate-language.mdx": "reference/predicate-language",
  "reference.mdx": "reference/reference",
  "resources.mdx": "reference/resources",
  "signals.mdx": "reference/signals",
  "signature-algorithms.mdx": "reference/signature-algorithms",
  "user-types.mdx": "reference/user-types",
  "access-controls": {
    "access-controls.mdx": "reference/access-controls/access-controls",
    "access-lists.mdx": "reference/access-controls/access-lists",
    "authentication.mdx": "reference/access-controls/authentication",
    "login-rules.mdx": "reference/access-controls/login-rules",
    "roles.mdx": "reference/access-controls/roles",
    "saml-idp.mdx": "reference/access-controls/saml-idp"
  },
// [...]

[ptgott]

I propose we:

• Add a link for each top-level sidebar page (https://docusaurus.io/docs/next/sidebar/items#sidebar-item-doc)
• Customize the sidebar item generator (https://docusaurus.io/docs/next/sidebar/autogenerated#customize-the-sidebar-items-generator) to, first, call the default sidebar generator function, then delete any items that are top-level links, avoiding the duplication.

[ptgott]

A related issue: https://goteleport.com/docs/connect-your-client renders without a sidebar.