feat(theme): dark mode by default, with a light/dark toggle

The docs site only offered Material's light scheme. This adds the dark
scheme and makes it the default.

- palette: two entries, slate first, each with a toggle. Material takes the
  first entry as the default, so the site opens dark and a sun icon in the
  header switches to light (and a moon switches back).
- dc-brand.css: the dark scheme's greys are derived from a hue variable, so
  it is pointed at the brand slate (206deg) rather than Material's neutral
  225deg, which gives the whole scheme the same cast as the header.
- dc-brand.css: the brand slate is unreadable on a dark background, so the
  dark scheme uses a light tint of it (#9fb5c6) for body links. Green stays
  the accent, which keeps link hover visible in both schemes.

Contrast, over the dark background rgb(30, 36, 41): body text 8.8:1, body
links 7.4:1, inline code 7.2:1, accent green 5.6:1, header text 10.2:1. The
light scheme is unchanged.

mkdocs.yml

@@ -116,9 +116,18 @@ theme:
   name: material
   logo: 'img/dc-logo.svg'
   palette:
+    - scheme: slate
+      primary: custom
+      accent: custom
+      toggle:
+        icon: material/weather-sunny
+        name: Switch to light mode
     - scheme: default
       primary: custom
       accent: custom
+      toggle:
+        icon: material/weather-night
+        name: Switch to dark mode
   font:
     text: 'Montserrat'
     code: 'Ubuntu Mono'