doc: add missing nav

Author: domenkozar

docs/src/.nav.yml

@@ -5,6 +5,7 @@ nav:
     - Getting started: getting-started.md
     - Files and variables: files-and-variables.md
     - Ad-hoc environments: ad-hoc-developer-environments.md
+    - Auto activation: auto-activation.md
     - Writing devenv.nix:
       - Basics: basics.md
       - Packages: packages.md