feat(docs): "?" help affordance on every filter mount + docs loader fixes

The filter manual (1ed1602e) was reachable only by knowing /docs
already existed — nothing in the UI pointed at it (the parked manual
plan's open decision). Every filter-builder mount now carries a "?"
in its header linking to the guide (opens in a new tab so drawer and
editor state survive): the All Books drawer and the collection rules
page — the two mounts sharing FilterBuilderRows. The docs home
indexes the manual (guide, cookbook, generated field reference).

Two docs-loader fixes surfaced by pinning this:
- Folder URLs resolve to their index page: /docs/user/filters now
  serves user/filters/index.md (the manual is a folder: guide +
  cookbook + generated reference); previously only root /docs had
  index handling.
- The docs folder path is resolved by walking upward (bounded):
  `go test` runs test binaries from the PACKAGE directory, so the
  relative "docs" path resolved to a folder that didn't exist there
  and every /docs route 404'd in tests while working in prod —
  found by the new pin, fixed so tests exercise the docs routes
  exactly as prod serves them.

Pins: TestFilterHelpAffordance (folder URL serves the guide; drawer
and rules pages carry the link). internal/docs suite + full suite
green (239s).

templates/collection_rules_templ.go

@@ -219,7 +219,7 @@ func CollectionRules(user User, collection CollectionData, fieldsJSON string, tr
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 18, "\" class=\"card p-6\"><div class=\"flex items-center justify-between mb-3 px-1\"><h2 class=\"text-lg font-bold\" style=\"color: var(--text-primary)\">Filter tree</h2></div>")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 18, "\" class=\"card p-6\"><div class=\"flex items-center justify-between mb-3 px-1\"><h2 class=\"text-lg font-bold\" style=\"color: var(--text-primary)\">Filter tree</h2><a href=\"/docs/user/filters\" target=\"_blank\" rel=\"noopener\" class=\"icon-btn\" aria-label=\"Filter guide\" title=\"How the filter tree works\">?</a></div>")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}