docs site: legible header in dark mode, typography in its own stylesheet

Dark mode: the slate scheme set the header to #3f6b3e, a dark green on a dark page, so
the bar disappeared into the background. It is now green-mid with near-black text.

Typography moves to publishing/typography.css, loaded after brand.css, and declares
Figtree on the elements themselves -- Material defines --md-text-font-family on <body>,
so a :root value loses to it by DOM proximity. Heading colours are scoped to the light
scheme, since ink is unreadable on the dark one.

mkdocs.yml

@@ -107,6 +107,7 @@ markdown_extensions:
 extra_css:
   # Co-op brand: palette from the website + self-hosted Figtree (SIL OFL 1.1).
   - publishing/brand.css
+  - publishing/typography.css
 
 # Keep build/tooling files out of the rendered site.
 exclude_docs: |