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/sections/FeatureGrid.tsx

@@ -0,0 +1,15 @@
+import type { Feature } from "@rspress/core"
+import { HomeFeature } from "@rspress/core/theme-original"
+import { arr } from "./types"
+
+/**
+ * Card grid used for both the homepage advantages and the products/services
+ * grids. Delegates to the stock `HomeFeature`, which already handles `span`,
+ * icons and the reveal animation — it accepts a `features` prop precisely so it
+ * can be driven from somewhere other than page frontmatter.
+ */
+export function FeatureGrid({ items }: { items?: Feature[] }) {
+  const features = arr<Feature>(items)
+  if (!features) return null
+  return <HomeFeature features={features} />
+}