fix(docs): folder-index pages expand the sidebar and highlight — same rule for every page

The filter manual is the docs tree's first FOLDER-index page: it serves
at /docs/user/filters while its nav entry carries /index
(/docs/user/filters/index). The sidebar's two checks — group expansion
(containsURL) and active-link styling (docsNavClass) — compared URLs
EXACTLY, so the folder URL matched neither: no submenu expansion, no
highlight (2026-11-01 user report: "the left sidebar should open to
the submenu and highlight the page").

Fix is structural, not per-page: both checks normalize folder-index
URLs (a trailing /index is the same page as its directory), so EVERY
folder-index page — current and future — expands and highlights like
a file page. No per-page registration exists anywhere in the docs
system to redo: the sidebar derives from the docs tree, and this was
the one URL shape its equality rule missed.

Pins: navigation (section + group expand for the folder URL, group
present) and docsNavClass (folder URL highlights the Overview item,
normalization symmetric, file pages keep their highlight, no
cross-highlighting).

internal/docs/navigation.go

@@ -321,10 +321,23 @@ func (h *DocsHandler) BuildNavigation(currentPath string) *templates.Navigation
 	return nav
 }
 
-// containsURL reports whether currentPath is one of the given doc URLs.
+// normalizeDocURL — folder-index pages SERVE at their directory URL
+// (/docs/user/filters) while their nav entry carries the /index suffix
+// (/docs/user/filters/index): same page, two spellings. Both sidebar
+// checks (expansion + highlight) compare normalized forms, so folder
+// pages expand and highlight like file pages — no per-page special
+// cases (2026-11-01, the filter manual's folder).
+func normalizeDocURL(u string) string {
+	u = strings.TrimRight(u, "/")
+	return strings.TrimSuffix(u, "/index")
+}
+
+// containsURL reports whether currentPath is one of the given doc URLs
+// (folder-index URLs normalized — see normalizeDocURL).
 func containsURL(currentPath string, urls []string) bool {
+	current := normalizeDocURL(currentPath)
 	for _, u := range urls {
-		if currentPath == u {
+		if current == normalizeDocURL(u) {
 			return true
 		}
 	}