Make better use of sections

[ChrisBAshton]

Every doc has a section: <foo> in its metadata, which in theory could be useful for grouping said docs. But this information isn't obviously exposed in the frontend, apart from in the "More in the section" at the bottom of each page.

I imagine exposing the sections on the homepage would:

1. Expose which sections are actually defined (making it easier to identify themes and consolidate)
2. Allow a way of navigating by click (no search), which is an accessibility requirement we're probably failing at at the moment

[nicholsj]

The sections are displayed in the left navigation bar of https://docs.publishing.service.gov.uk/manual.html - but yes agree they could be better used.

[ChrisBAshton]

🤦 good spot - thanks!

[ChrisBAshton]

Spotted a bug for docs that don't have a 'section' specified:

We should probably add a quick CI check that fails if any added document is missing a 'section'