Drop the manual index bookkeeping

Make a page navigable and correctly ordered without hand-managed nav_order and in_nav front matter.

done

Adding a page used to mean remembering in_nav: true, picking a free nav_order, and setting title and category by hand. Forgetting any of them failed silently: the page still built, but disappeared from the sidebar, search, the graph, and the directory at once.

What changed

Navigable by default. A page is in the nav unless it sets in_nav: false, or is a section index. The old rule was the inverse.
nav_order is optional. Pages that set it come first, in that order; everything else follows alphabetically by title.
title and category are optional. Title falls back to the URL slug, category to the section label.
One definition, not seven. Every consumer — sidebar, search, graph, directory, tags, categories, the section redirect — now goes through _includes/nav-docs.html instead of walking the collections itself.

A new page needs an empty front matter block and nothing else.