docs(site): add custom 404 page with navigation links

Created a custom Starlight 404 page (docs/manual/404.md) that includes LinkCards pointing to common sections (Installation, Troubleshooting, Function Reference). Also updated build-manual.py to prevent '404' from appearing in the generated sidebar.

docs/build-manual.py

@@ -673,7 +673,7 @@ def build_site(root: Path, out: Path) -> list[dict]:
             target.parent.mkdir(parents=True, exist_ok=True)
             body = _inject_subheading_cards(body)
             _write_prettified(target, _page_fm(fm), prettify(body))
-            if rel.name != "index.md":
+            if rel.name not in ("index.md", "404.md"):
                 sidebar.append({"label": fm["title"], "link": "/" + rel.stem + "/"})
             continue
 

docs/manual/404.md

@@ -0,0 +1,15 @@
+---
+title: '404'
+template: splash
+editUrl: false
+man: false
+hero:
+  title: '404'
+  tagline: Page not found. Check the URL or try using the search bar.
+---
+
+<CardGrid>
+  <LinkCard title="Installation" description="Set up the configuration" href="/09-installation/" />
+  <LinkCard title="Troubleshooting" description="Fix common issues" href="/11-troubleshooting/" />
+  <LinkCard title="Function Reference" description="Browse all functions" href="/05-functions/" />
+</CardGrid>