style(theme): apply the Data Controller brand palette and logo

The docs site rendered as stock Material: indigo header, indigo links and
Open Sans. Neither colour nor type matched the marketing site
(datacontroller.io) or the application itself.

- Palette: the slate (#314351) and green (#79a843) already used by the
  marketing navbar, hero and logo accent, wired up via a `custom` palette
  and defined in the new docs/dc-brand.css.
- Type: Montserrat for text, matching the marketing site's headings.
- Logo: the application's own dc-logo.svg mark in the header.

The previous `palette:` mapping form (a dict of primary/accent) is no
longer honoured by the Material version this repo builds against, so the
theme fell through to Material's default indigo - the site was not in fact
rendering the White/Amber it asked for. The list form used here is the one
Material currently reads.

mkdocs.yml

@@ -96,7 +96,9 @@ markdown_extensions:
 extra:
   manifest: manifest.webmanifest
 
-extra_css: ['font-awesome.css']
+extra_css:
+  - font-awesome.css
+  - dc-brand.css
 
 plugins:
   - search:
@@ -112,12 +114,13 @@ repo_url: 'https://git.datacontroller.io/dc/docs.datacontroller.io'
 
 theme:
   name: material
-  logo: 'img/favicon.ico'
+  logo: 'img/dc-logo.svg'
   palette:
-    primary: 'White'
-    accent: 'Amber'
+    - scheme: default
+      primary: custom
+      accent: custom
   font:
-    text: 'Open Sans'
+    text: 'Montserrat'
     code: 'Ubuntu Mono'
   favicon: img/favicon.ico
   custom_dir: 'theme'