docs: simplify navigation and layout

docs/theme/layout.css

@@ -1,7 +1,4 @@
-/*
- * A static documentation layout: kernel-docs navigation with lore-like
- * restraint. Colours follow Orkavian's black, silver, and green palette.
- */
+/* Static documentation layout. */
 :root,
 html.light,
 html.rust,
@@ -80,16 +77,6 @@ body {
     margin: 0;
 }
 
-.sidebar-scrollbox .part-title {
-    color: var(--kaptan-muted);
-    font-size: 0.75rem;
-    font-weight: 700;
-    letter-spacing: 0.08em;
-    margin: 0;
-    padding: 1rem 1rem 0.3rem;
-    text-transform: uppercase;
-}
-
 .sidebar-scrollbox .chapter-item a {
     color: var(--kaptan-text);
     display: block;
@@ -123,7 +110,12 @@ body {
     box-sizing: border-box;
     margin: 0 auto;
     max-width: 78ch;
-    padding: 2rem 1.5rem 3rem;
+    padding: 4rem 1.5rem 3rem;
+}
+
+/* mdBook offsets this for its menu placeholder; this layout hides that menu. */
+.page {
+    margin-block-start: 0 !important;
 }
 
 .content main {
@@ -202,8 +194,8 @@ code {
 }
 
 pre {
-    background: var(--kaptan-surface);
-    border: 1px solid var(--kaptan-border);
+    background: #1d1712;
+    border: 1px solid #5b4935;
     overflow-x: auto;
     padding: 0;
 }