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.

docs/dc-brand.css

@@ -0,0 +1,27 @@
+/*
+ * Data Controller brand overrides for the Material theme.
+ *
+ * The values below mirror the marketing site (https://datacontroller.io): the
+ * slate used for its navbar and hero, and the green used for its logo accent.
+ * They are wired up through the `custom` palette in mkdocs.yml, which leaves
+ * these CSS variables for us to define.
+ */
+
+/* Primary - the brand slate, used for the header, the mobile drawer title and body links. */
+:root,
+[data-md-color-primary='custom'] {
+  --md-primary-fg-color: #314351;
+  --md-primary-fg-color--light: #3d5468;
+  --md-primary-fg-color--dark: #263542;
+  --md-primary-bg-color: #fff;
+  --md-primary-bg-color--light: #ffffffb3;
+  --md-typeset-a-color: #314351;
+}
+
+/* Accent - the brand green, used for link hover, focus rings and the active search hit. */
+[data-md-color-accent='custom'] {
+  --md-accent-fg-color: #79a843;
+  --md-accent-fg-color--transparent: #79a8431a;
+  --md-accent-bg-color: #fff;
+  --md-accent-bg-color--light: #ffffffb3;
+}