feat: platform logos and emulator letter sections

The decoration pass now forwards the logo front matter, pointed at the
local copy. The emulators index lists every profile under the initial
the sidebar draws, so each letter link lands on its section.

scripts/sitestarlight.py

@@ -406,6 +406,7 @@ def finalize_page(
     structured: dict,
     terms: dict[str, str],
     search: bool = True,
+    logo: str = "",
 ) -> tuple[str, str]:
     """The Starlight file for a page body: its path under the content
     collection and its text."""
@@ -446,6 +447,8 @@ def finalize_page(
         front["slug"] = stem
     if level:
         front["level"] = level
+    if logo:
+        front["logo"] = logo
     if weight:
         front["searchWeight"] = weight
     if rel == NOT_FOUND: