Document .mkdocsignore path exclusion in file-filter plugin

Explain that `.mkdocsignore` prunes whole sections and follows the same
`enabled` gate as label filtering, so dev-only areas stay visible during
development/serve and are hidden only in production. Note the
gitignore-like fallback when no `.file-filter.yml` is present.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Netdocs

docs-site/docs/plugins/file-filter.md

@@ -22,6 +22,10 @@ enabled_on_serve: true
 # Front-matter property that holds the labels.
 metadata_property: labels
 
+# Path-based exclusion via a .mkdocsignore file (see below).
+mkdocsignore: true
+mkdocsignore_file: .mkdocsignore
+
 # Pages with any of these labels are excluded…
 exclude_tag:
   - draft
@@ -44,13 +48,37 @@ labels:
 This page is excluded when the filter is active.
 ```
 
+## Path exclusion with `.mkdocsignore`
+
+For whole sections you don't want in production, list their paths in a **`.mkdocsignore`**
+at the project root (same glob style as `.gitignore`):
+
+```text title=".mkdocsignore"
+# Dev-only areas — present locally, hidden in production
+internal-notes/
+teams/
+```
+
+Path exclusion follows the **same `enabled` gate** as label filtering, so those sections
+stay visible during development / `serve` and disappear only from production builds. The
+filter turns on when `enabled` resolves true — drive it from a build flag, e.g.
+`enabled: !ENV [MKDOCS_PROD_BUILD, false]` (Netdocs sets `MKDOCS_PROD_BUILD=true` on
+`--prod`).
+
+!!! note "No `.file-filter.yml`?"
+    If you have a `.mkdocsignore` but **no** `.file-filter.yml`, the ignore file is always
+    applied (like `.gitignore`). The gate only exists once you opt into the filter.
+
 ## Behaviour
 
 - The filter is **active** when `enabled` is true (on `serve`, `enabled_on_serve` must
   also be true).
+- While active, `.mkdocsignore` paths are pruned from discovery; while inactive they are
+  kept, so dev-only sections reappear on non-production builds.
 - An **include** label always keeps a page.
 - Otherwise, any **exclude** label prunes the page from discovery and navigation.
-- If no `exclude_tag` values are configured, the filter is a no-op.
+- If no `exclude_tag` values are configured, the **label** filter is a no-op (path
+  exclusion via `.mkdocsignore` still applies when active).
 
 ## Enabling the plugin