Navigation breaks after two levels of depth

left-hand menu and link path at top of page doesn't work well for pages lower than depth two. Fix or, if we have to, reorganise to avoid.

Looks like a nicer fix for this is coming once https://github.com/pmarsceill/just-the-docs/pull/192 has been merged into JTD

For now, I've added the grand_parent field to the front matter of all relevant sub-section pages in 9ee7be11

This can removed, along with the parent field, if the PR linked above (or something similar) is included in JTD