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/SectionBand.tsx

@@ -0,0 +1,49 @@
+import type { ReactNode } from "react"
+import { SHOW_COPY_PLACEHOLDERS } from "./placeholder"
+import { str, type Section } from "./types"
+
+interface SectionBandProps {
+  section: Section
+  /** Whether the section body produced any content. */
+  hasContent: boolean
+  children: ReactNode
+}
+
+/**
+ * Shell for a marketing band: the `<section>` wrapper, its heading and its
+ * subtitle. Renders nothing at all when the section has neither content nor a
+ * heading, so an unfilled page degrades to just its hero.
+ */
+export function SectionBand({ section, hasContent, children }: SectionBandProps) {
+  const title = str(section.title)
+  const subtitle = str(section.subtitle)
+
+  if (!hasContent && !title && !subtitle) {
+    if (!SHOW_COPY_PLACEHOLDERS) return null
+    return (
+      <section id={section.id} className="rp-site-section">
+        <div className="mx-auto max-w-5xl px-6">
+          <div className="rounded-lg border border-dashed border-(--rp-c-divider) p-8 text-center text-sm text-(--rp-c-text-3)">
+            TODO: {section.type}
+          </div>
+        </div>
+      </section>
+    )
+  }
+
+  return (
+    <section id={section.id} className="rp-site-section">
+      <div className="mx-auto max-w-5xl px-6">
+        {(title || subtitle) && (
+          <header className="mb-10 text-center">
+            {title && <h2 className="m-0 text-3xl font-semibold text-(--rp-c-text-1)">{title}</h2>}
+            {subtitle && (
+              <p className="mx-auto mt-3 max-w-2xl text-base text-(--rp-c-text-2)">{subtitle}</p>
+            )}
+          </header>
+        )}
+        {children}
+      </div>
+    </section>
+  )
+}