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/components/HomeSections.tsx

@@ -0,0 +1,63 @@
+import { useFrontmatter } from "@rspress/core/runtime"
+import { SectionBand } from "./sections/SectionBand"
+import { FeatureGrid } from "./sections/FeatureGrid"
+import { TechStack } from "./sections/TechStack"
+import { Cases } from "./sections/Cases"
+import { CallToAction } from "./sections/CallToAction"
+import { hasContent, type Section } from "./sections/types"
+
+/** Type guard for a single entry of `frontmatter.sections`. */
+function isSection(value: unknown): value is Section {
+  if (typeof value !== "object" || value === null) return false
+  const type = (value as { type?: unknown }).type
+  return (
+    type === "advantages" ||
+    type === "contentGrid" ||
+    type === "techStack" ||
+    type === "cases" ||
+    type === "cta"
+  )
+}
+
+function renderBody(section: Section) {
+  switch (section.type) {
+    case "advantages":
+    case "contentGrid":
+      return <FeatureGrid items={section.items} />
+    case "techStack":
+      return <TechStack groups={section.groups} />
+    case "cases":
+      return <Cases items={section.items} />
+    case "cta":
+      return <CallToAction actions={section.actions} code={section.code} />
+  }
+}
+
+/**
+ * Renders `frontmatter.sections` in declaration order.
+ *
+ * Attached to the `Layout` `afterHero` (home) and `afterDoc` (products,
+ * services) slots. Returns `null` for any page without a `sections` array, so
+ * it is inert on blog posts, project pages and every other doc page.
+ */
+export function HomeSections() {
+  const { frontmatter } = useFrontmatter()
+  const raw = frontmatter?.sections
+  if (!Array.isArray(raw)) return null
+
+  const sections = raw.filter(isSection)
+  if (sections.length === 0) return null
+
+  return (
+    <div className="rp-site-sections">
+      {sections.map((section, index) => (
+        <SectionBand
+          key={section.id ?? `${section.type}-${index}`}
+          section={section}
+          hasContent={hasContent(section)}>
+          {renderBody(section)}
+        </SectionBand>
+      ))}
+    </div>
+  )
+}