docs: small component documentation

Author: gsiri-code

docs/Coordinates.md

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+# Design Project Assignment : Small Component:
+
+The data layer now includes a small `data.coordinates` component for validating city coordinates before writes reach MongoDB.
+
+## What was added
+
+### `feat: create small component for data.coordinates`
+
+`data/coordinates.py` provides:
+
+- `Coordinate` — abstract base class for validated numeric coordinate values
+- `Latitude` — validates values in the range `-90..90`
+- `Longitude` — validates values in the range `-180..180`
+- `Coordinates` — wrapper for paired latitude/longitude values with:
+  - `from_dict()`
+  - `to_dict()`
+
+Example:
+
+```python
+from data.coordinates import Coordinates
+
+coords = Coordinates.from_dict({
+    "latitude": 39.78,
+    "longitude": -89.64,
+})
+
+payload = coords.to_dict()
+```
+
+## Where it is used
+
+`data/cities.py` now validates coordinate payloads in:
+
+- `add_city()`
+- `update_city()`
+- `update_city_by_name_and_country()`
+
+The stored data shape is unchanged:
+
+```python
+{
+    "coordinates": {
+        "latitude": 39.78,
+        "longitude": -89.64,
+    }
+}
+```
+
+## Validation behavior
+
+The component rejects:
+
+- non-dict coordinate payloads
+- missing `latitude`
+- missing `longitude`
+- non-numeric values
+- boolean values
+- latitude outside `-90..90`
+- longitude outside `-180..180`
+
+## Tests
+
+### `test: add testing to coordinates component`
+
+Coverage was added in:
+
+- `data/tests/test_coordinates.py`
+- `data/tests/test_cities.py`
+
+Run the relevant tests with:
+
+```bash
+python -m pytest data/tests/test_coordinates.py data/tests/test_cities.py -q
+```