Add RTC HAL page to PG

Author: bulislaw

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 located in [hal/rtc_api.h](). Following functions need to be implemented to support RTC:
+
+
+To enable sleep support in mbed OS `RTC` label needs to be added in `device_has` option of target's section in `targets.json` file.
+
+## Testing
+
+The [RTC HAL API test suite]() validates:
+
+- `rtc_init` is safe to call repeatedly - Verified by test `rtc_init_test`
+- `Init`/`free` doesn't stop RTC from counting - Verified by test `rtc_persist_test`
+- Software reset doesn't stop RTC from counting - Verified by `rtc_reset_test`
+- Sleep modes don't stop RTC from counting - Verified by `rtc_sleep_test`
+- Overflow handling - Verified by `rtc_range_test`
+- No backward glitches - Verified by `rtc_glitch_test`