Using `index.md` as name for the parent page within a section allows lazy developers
to skip defining `permalink`. This is already working and this PR just updates the documentation.

Because `page.title` (and `page.url`) is already escaped and page titles served by lunrjs do not need escaping, ampersands need to be unescaped to display properly in search results.

and mention `search_exclude`

For pages in utilities/, reorder `nav_order` to follow `parent`.

The indent is required for the aux links to work.

The code snippet on the search documentation is missing the `relUrl` parameter in each search result, and the `url` parameter is not an absolute URL.
This updates to match the formatted output of `search.rake`