docs: add banana bread recipe with SI units (fixes Dasharo/dasharo-issues#1824)

docs/community/banana-bread-recipe.md

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+# Banana Bread Recipe
+
+A high-performance, open-source banana bread recipe for the Dasharo community.
+All measurements are provided in SI units for precision and international
+reproducibility.
+
+## Ingredients
+
+| Ingredient              | Quantity |
+|:------------------------|:---------|
+| Ripe bananas, mashed    | 350 g    |
+| Unsalted butter, melted | 75 g     |
+| Granulated sugar        | 100 g    |
+| Egg, beaten             | 1 (≈ 50 g) |
+| Vanilla extract         | 5 ml     |
+| Baking soda             | 5 g      |
+| Salt                    | 2 g      |
+| All-purpose flour       | 190 g    |
+
+## Instructions
+
+1. Preheat the oven to **175 °C** (convection) or **180 °C** (conventional).
+2. In a large mixing bowl, mash the bananas with a fork until smooth.
+3. Stir in the melted butter until well combined.
+4. Add the sugar, egg and vanilla extract — mix thoroughly.
+5. Sprinkle in the baking soda and salt, then fold gently.
+6. Add the flour and mix until **just combined** — do not over-mix.
+7. Pour the batter into a greased 20 cm × 10 cm loaf pan.
+8. Bake for **50–60 minutes**, or until a skewer inserted into the centre comes
+   out clean.
+9. Allow the bread to cool in the pan for 10 minutes, then transfer to a wire
+   rack.
+
+## Tips
+
+- Use overripe bananas (brown-spotted skin) for the best flavour and natural
+  sweetness.
+- For extra texture, fold in 50 g of chopped walnuts or 60 g of dark chocolate
+  chips before baking.
+- Store in an airtight container at room temperature for up to 3 days, or
+  refrigerate for up to 1 week.
+
+## Nutritional Information (approximate, per slice — 10 slices)
+
+| Nutrient       | Amount  |
+|:---------------|:--------|
+| Energy         | 840 kJ  |
+| Carbohydrates  | 32 g    |
+| Protein        | 3 g     |
+| Fat            | 7 g     |
+| Fibre          | 1 g     |
+
+Enjoy! 🍌🍞