add docs and examples

docs/src/tutorial/units.md

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+# Units
+
+Each recipe ingredient has a string unit assigned to it. This is great, but a centralized list of all alowed units with some additional properties has several advantages:
+
+* spelling mistakes are not silently ignored, if you make a mistake in the ingredient unit name, comfyrecipes will warn you that it's not on the units list
+* `gram` and `g` can refer to the same unit (using aliases)
+* we can tell comfyrecipes unit conversion rates (for example 1000 gram = 1 kilogram) which it can then use for example for calculating prices
+
+This example will use the final example from [Quick Start](quickstart.md).
+
+`🗎 myrecipes/recipes/pancakes.yaml:`
+```yaml
+{{#include ../../examples/firstrealrecipe.yaml}}
+```
+
+```
+- myrecipes/
+    - recipes/
+        - pancakes.yaml
+    ...
+    - units.yaml (we will be creating this file)
+```
+
+If you don't want to write the file manually, we can generate it using:
+```sh
+$ comfyrecipes generate-units
+```
+
+This will generate a minimal units.yaml:
+
+`🗎 myrecipes/units.yaml:`
+```yaml
+- name: cup
+- name: piece
+- name: tablespoon
+```
+
+Now, let's say we would want to call `tablespoon` just `tbsp`, we can add an alias:
+
+`🗎 myrecipes/units.yaml:`
+```yaml
+- name: cup
+- name: piece
+- name: tablespoon
+  aliases:
+    - tbsp
+```
+
+Now we can rename the `tablespoon` unit in the recipe to `tbsp` and it will reference the `tablespoon` unit.
+
+For a full reference for what a recipe yaml can contain, please see the [Units Reference](../reference/units.md)