docs: add middleware to layers guide

Author: danielroe

docs/2.guide/3.going-further/7.layers.md

@@ -18,6 +18,7 @@ Additionally, certain other files in the layer directory will be auto-scanned an
 - [`components/*`](/docs/guide/directory-structure/components)   - Extend the default components
 - [`composables/*`](/docs/guide/directory-structure/composables)  - Extend the default composables
 - [`layouts/*`](/docs/guide/directory-structure/layouts)  - Extend the default layouts
+- [`middleware/*`](/docs/guide/directory-structure/middleware)  - Extend the default middleware
 - [`pages/*`](/docs/guide/directory-structure/pages)        - Extend the default pages
 - [`plugins/*`](/docs/guide/directory-structure/plugins)        - Extend the default plugins
 - [`server/*`](/docs/guide/directory-structure/server)       - Extend the default server endpoints & middleware