add a tutorial on packaging existing software

source/tutorials/learning-journey/index.md

@@ -1,10 +1,12 @@
+(learning-journey)=
 # Learning Journey
 
 This collection of tutorials guides you through your first steps with Nix.
 This series is a work in progress and will have some overlap with existing tutorials.
 The intention is to unify these tutorials over time.
 
- ```{toctree}
- :maxdepth: 1
- shell-dot-nix.md
- ```
+```{toctree}
+:maxdepth: 1
+packaging-existing-software.md
+shell-dot-nix.md
+```