Improve the sidebar generator

Improve the sidebar generator to accommodate the reorganized docs site.

Alphabetize auto-generated sidebar entries.

The exception is any page that includes "Introduction" or "introduction" in the title. Elevate these to the first entry to avoid a confusing sidebar order.
Show third-level category pages in the sidebar without showing their contents. This way, we can add content one level beyond the level permitted in the sidebar while still showing the category page for that content in the sidebar.
Allow for a category page to be at the same level as its associated subdirectory. This is to accommodate the Terraform Provider reference which, because of the way it's generated, can only include a category page outside of its associated subdirectory. This does not abide by the Docusaurus convention, so we will need to figure out a solution eventually, but this is a quick alternative to tide us over.

gravitational / docs