feat: write the site for Starlight behind a flag

generate_site.py --engine starlight writes the pages, navigation and
data files of an Astro Starlight project in website/. Links resolve to
site addresses, every heading keeps its MkDocs id, and pages split by
size are measured in the MkDocs Markdown so both builds cut the same
files. The 839 pages and their 45,613 ids match the MkDocs build.

scripts/siterender.py

@@ -7,6 +7,8 @@ import urllib.parse
 from collections.abc import Callable
 from pathlib import Path
 
+import sitemarkup as markup
+
 
 WIKI_SRC_DIR = "wiki"  # manually maintained wiki sources
 SYSTEM_ICON_BASE = "https://raw.githubusercontent.com/libretro/retroarch-assets/master/xmb/systematic/png"
@@ -89,7 +91,7 @@ def system_icon_markdown(manufacturer: str, console_name: str) -> str:
     name = _icon_name(manufacturer, console_name)
     if not _icon_available.get(name):
         return ""
-    return f"![{console_name}]({_icon_url(name)}){{ width=24 }} "
+    return markup.image(console_name, _icon_url(name), width="24") + " "
 
 # Global index: maps system_id -> (manufacturer_slug, console_name) for cross-linking
 _system_page_map: dict[str, tuple[str, str]] = {}