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/sitemarkup.py

@@ -13,6 +13,7 @@ from __future__ import annotations
 
 import html
 import re
+from contextlib import contextmanager
 
 ENGINES = ("mkdocs", "starlight")
 _engine = "mkdocs"
@@ -39,6 +40,19 @@ def engine() -> str:
     return _engine
 
 
+@contextmanager
+def using(name: str):
+    """Write in another engine's syntax for a while. A page split by size
+    is measured in the MkDocs Markdown its budgets were tuned on, so both
+    engines cut the same pages at the same files."""
+    previous = _engine
+    set_engine(name)
+    try:
+        yield
+    finally:
+        set_engine(previous)
+
+
 def starlight() -> bool:
     return _engine == "starlight"