Add RTC HAL page to PG

docs/contributing/rtc.md

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+## RTC HAL API
+
+The RTC HAL API provides a low-level interface to the Real Time Counter (RTC) of a target.
+
+### Assumptions
+
+#### Defined behavior
+
+- The function `rtc_init` is safe to call repeatedly.
+- RTC accuracy is at least 10%.
+- `Init`/`free` doesn't stop RTC from counting.
+- Software reset doesn't stop RTC from counting.
+- Sleep modes don't stop RTC from counting.
+- Shutdown mode doesn't stop RTC from counting.
+
+#### Undefined behavior
+
+- Calling any function other than `rtc_init` before the initialization of the RTC.
+
+#### Potential bugs
+
+- Incorrect overflow handling.
+- Glitches due to ripple counter.
+
+### Implementing the RTC API
+
+RTC HAL API is in [hal/rtc_api.h](/docs/v5.4/mbed-os-api-doxy/rtc__api_8h_source.html). You need to implement the following functions to support RTC:
+
+To enable sleep support in Mbed OS, you need to add the `RTC` label in the `device_has` option of the target's section in the `targets.json` file.