docs: centralize Markdown developer manual

docs/theme/kaptan.css

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+:root {
+  --kaptan-ink: #142033;
+  --kaptan-blue: #1769aa;
+  --kaptan-sky: #e8f3fb;
+  --kaptan-border: #d4dfeb;
+}
+
+.content main {
+  max-width: 58rem;
+  margin: 0 auto;
+}
+
+.content h1,
+.content h2,
+.content h3 {
+  color: var(--kaptan-ink);
+  font-weight: 700;
+  letter-spacing: -0.02em;
+}
+
+.content h1 {
+  border-bottom: 3px solid var(--kaptan-blue);
+  padding-bottom: 0.45rem;
+}
+
+.content a {
+  color: var(--kaptan-blue);
+}
+
+.content table {
+  border: 1px solid var(--kaptan-border);
+  border-radius: 0.5rem;
+  overflow: hidden;
+}
+
+.content blockquote {
+  border-left-color: var(--kaptan-blue);
+  background: var(--kaptan-sky);
+}
+
+.sidebar .sidebar-scrollbox {
+  border-right: 1px solid var(--kaptan-border);
+}
+
+.nav-chapters {
+  border-top: 1px solid var(--kaptan-border);
+}