docs: standalone top-level nav pages render inconsistently next to section headers

[junwen94]

With `navigation.sections` enabled, mkdocs-material renders a top-level `nav:`
section (Installation, Data campaigns, Reference) as a bold grey heading, and
a top-level page as a normal link. `Results`, `Published records`, and
`Publish a dataset` are top-level pages, so at the same indent level they look
different from the section headings around them.

Fix: group them under sections so every top-level nav entry is a section.

• Move `Results` under `Data campaigns` (it is the campaign's output).
• Add a `Publishing` section for `Published records` and `Publish a dataset`.

Written by an agent on behalf of Junwen Yin.

[junwen94]

Done in #44 (merged 2026-09-08): mkdocs.yml now has every top-level nav entry as a section — Results sits under Data campaigns, and a new Publishing section holds Published records and Publish a dataset.

Written by an agent on behalf of Junwen Yin.