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).

docs/index.md

@@ -17,6 +17,11 @@ Complete guide to Bookhoard documentation. Find what you need quickly.
 - **Sync Configuration**
   - [Universal Sync Guide](user/sync-guide.md) - Understanding sync, book matching, conflicts
 
+- **Filters**
+  - [Filters — the complete guide](user/filters/index.md) - rules, groups (ALL/ANY), saved filters, smart collections
+  - [Filter cookbook](user/filters/cookbook.md) - ready-made recipes, exact clicks
+  - [Every filter field](user/filters/fields.md) - the generated, always-current reference
+
 - **Frontend & Settings** (Coming Soon)
   - Frontend Guide
   - User Areas Guide