feat(theme): add site footer and marketing sections

- Add SiteFooter, rendered through the Layout bottom slot so it appears on
  every page type. themeConfig.footer is deliberately left unset: setting
  it would render the built-in HomeFooter as well, on the home page only.
- Add a frontmatter-driven section system (advantages, contentGrid,
  techStack, cases and cta) with a single ordered array, so bands can be
  reordered or removed without touching code.
- Bands render only when they have content, so an unfilled page degrades
  to its heading rather than showing empty boxes.
- Add a SHOW_COPY_PLACEHOLDERS flag for drafting, false by default.

theme/index.tsx

@@ -1,8 +1,13 @@
 import "./index.css"
-import type { DocLayoutProps } from "@rspress/core/theme-original"
-import { DocLayout as OriginalDocLayout } from "@rspress/core/theme-original"
+import type { DocLayoutProps, LayoutProps } from "@rspress/core/theme-original"
+import {
+  DocLayout as OriginalDocLayout,
+  Layout as OriginalLayout,
+} from "@rspress/core/theme-original"
 import { Tags } from "./components/Tags"
 import { Author } from "./components/Author"
+import { SiteFooter } from "./components/SiteFooter"
+import { HomeSections } from "./components/HomeSections"
 
 function DocLayout(props: DocLayoutProps) {
   return (
@@ -24,6 +29,48 @@ function DocLayout(props: DocLayoutProps) {
   )
 }
 
+/**
+ * Wraps the stock `Layout` to add the site-wide footer and the frontmatter
+ * driven marketing sections.
+ *
+ * `bottom` is rendered for every page type (home, doc, doc-wide, custom and
+ * blank), so this is the one place the footer needs to be attached. Note that
+ * the original `Layout` resolves `DocLayout` from `@rspress/core/theme` — the
+ * alias for this directory — so the override above is picked up automatically.
+ *
+ * `afterFeatures` covers the home page and sits below the stock hero and
+ * features grid; `afterDoc` covers products, services and any other doc page
+ * that declares `sections`. `afterDoc` is a sibling of the doc container rather
+ * than part of the prose column, so sections placed there stay full-bleed.
+ * `HomeSections` renders nothing without the frontmatter, and the two slots are
+ * never both live on the same page.
+ */
+function Layout(props: LayoutProps) {
+  return (
+    <OriginalLayout
+      {...props}
+      bottom={
+        <>
+          {props.bottom}
+          <SiteFooter />
+        </>
+      }
+      afterFeatures={
+        <>
+          {props.afterFeatures}
+          <HomeSections />
+        </>
+      }
+      afterDoc={
+        <>
+          {props.afterDoc}
+          <HomeSections />
+        </>
+      }
+    />
+  )
+}
+
 export * from "@rspress/core/theme-original"
-export { DocLayout }
+export { DocLayout, Layout }
 export { Kbd } from "./components/Kbd"