Porting guide: Add Sleep API pg page

docs/contributing/sleep_deep_sleep.md

@@ -1,3 +1,52 @@
-### Sleep and deep sleep
+## Sleep HAL API
 
-[A document explaining sleep and deep sleep]
+Mbed OS defines two sleep modes for HAL:
+
+- Sleep.
+- Deep sleep.
+
+Each target should document in their implementation:
+
+- The target's mode description for each mode (how the target's mode maps to the Mbed OS sleep modes).
+- Wake-up latency for each mode.
+- Wake-up sources for each mode.
+
+### Sleep
+
+The core system clock is disabled, both the low and high precision clocks are enabled and RAM is retained.
+
+1. Wake-up sources - any interrupt source can wake up the MCU.
+1. Latency - can wake up within 10 us.
+
+### Deep sleep
+
+The core system clock is disabled. The low precision clocks are enabled, and RAM is retained.
+
+1. Wake-up sources - RTC, low power ticker and GPIO can wake up the MCU.
+1. Latency - can wake up within 10 ms.
+
+The deep sleep latency (10 ms) is the higher limit of the boards we support. Most of targets have wake-up latency for deep sleep within a few microseconds, but often, reinitializing clocks and other configurations require additional time to restore previous state.
+
+### Implementing the Sleep API
+
+There are two functions that the target needs to implement to support sleep, Their prototypes are in [hal/sleep_api.h](/docs/v5.4/mbed-os-api-doxy/sleep__api_8h_source.html):
+
+- Sleep.
+
+```c++
+void hal_sleep(void);
+```
+
+- Deep sleep.
+
+```c++
+void hal_deepsleep(void);
+```
+
+To enable sleep support in Mbed OS, you need to add the `SLEEP` label in the `device_has` option of the target's section in the `targets.json` file.
+
+### Testing
+
+The [sleep HAL API test suite]() validates:
+- Sleep wake-up sources.
+- Sleep wake-up latency.