docs(value-objects): add documentation for MedicationFrequency value object

docs/domain/value_objects/medication_frequency.md

@@ -0,0 +1,31 @@
+# `MedicationFrequency` -> `src/domain/value_objects/medication_frequency.rs`
+
+A value object representing how often a medication should be taken.
+
+---
+
+## Invariants
+
+- This follow an enum pattern with variants like `OnceDaily`, `TwiceDaily`, `EveryXHours(u8) or Custom(String)`.
+- For `EveryXHours`, the number of hours must be between 1 and 24
+
+
+## Usage
+
+```rust
+use bitpill::domain::value_objects::medication_frequency::MedicationFrequency;
+
+let freq1 = MedicationFrequency::OnceDaily;
+let freq2 = MedicationFrequency::EveryXHours(8);
+let freq3 = MedicationFrequency::Custom("Every Monday".to_string());
+
+assert_eq!(freq1.to_string(), "Once daily");
+assert_eq!(freq2.to_string(), "Every 8 hours");
+assert_eq!(freq3.to_string(), "Every Monday");
+```
+
+`MedicationFrequency` is used as a field in the [`Medication`](../entities/medication.md) entity to specify how often the medication should be taken.
+
+## Methods
+
+### `as_str() -> String`