docs: adopt the Spatial AI logo set and brand colours

.gitignore

@@ -9,7 +9,7 @@
 # Anchored: a bare c2t would also match the cmd/c2t source directory.
 /c2t
 city2tabula
-city2tabula-*
+/city2tabula-*
 /cmd/city2tabula
 build/
 bin/

README.md

@@ -1,4 +1,5 @@
-![city2tabula logo](docs/assets/logo/svg/city2tabula_logo_complete.svg)
+<picture><source media="(prefers-color-scheme: dark)" srcset="docs/assets/logo/city2tabula-lockup-violet-dark-outline.svg"><img alt="City2TABULA" src="docs/assets/logo/city2tabula-lockup-violet-outline.svg" width="360"></picture>
+
 [![go](https://github.com/thd-spatial-ai/city2tabula/actions/workflows/go.yml/badge.svg)](https://github.com/thd-spatial-ai/city2tabula/actions/workflows/go.yml)
 &nbsp;
 [![codecov](https://codecov.io/gh/thd-spatial-ai/city2tabula/branch/main/graph/badge.svg)](https://codecov.io/gh/thd-spatial-ai/city2tabula)

docs/stylesheets/extra.css

@@ -0,0 +1,32 @@
+/* Spatial AI brand colours: the header in brand ink, which the -dark mark is
+   drawn for, and City2TABULA violet for links and accents. */
+[data-md-color-scheme="default"] {
+  --md-primary-fg-color: #16191c;
+  --md-primary-fg-color--light: #2b2f33;
+  --md-primary-fg-color--dark: #0e1012;
+  --md-accent-fg-color: #6d4ae0;
+  --md-typeset-a-color: #6d4ae0;
+  --md-default-bg-color: #f7f5f1;
+}
+
+[data-md-color-scheme="slate"] {
+  --md-primary-fg-color: #16191c;
+  --md-primary-fg-color--light: #2b2f33;
+  --md-primary-fg-color--dark: #0e1012;
+  --md-accent-fg-color: #a08bff;
+  --md-typeset-a-color: #a08bff;
+  --md-default-bg-color: #16191c;
+}
+
+/* The header logo is the lockup, which carries the project name in brand
+   colours, so the plain-text site name is hidden where the logo shows. Below
+   76.25em Material moves the logo into the drawer and the text name stays. */
+.md-header__button.md-logo img {
+  height: 2rem;
+}
+
+@media screen and (min-width: 76.25em) {
+  .md-header__title .md-header__topic:first-child {
+    visibility: hidden;
+  }
+}