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

@@ -60,6 +60,7 @@ templ CollectionRules(user User, collection CollectionData, fieldsJSON string, t
 				>
 					<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>
 					@FilterBuilderRows("item")
 <div class="mt-4 pt-4 border-t flex justify-end gap-2" style="border-color: var(--border);">