[PHEE-631] Added API Doc for Bill Pay and Identity Account Mapper (#408)

* Added API Doc for Bill Pay and Identity Account Mapper

* API DOC Updated for Bill Pay and IDAM

* [PHEE-632] Add callback as an example in batch transaction API (#407)

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Co-authored-by: Yash Dwivedi <yash@Yashs-MacBook-Pro.local>
Co-authored-by: Abhinav Mishra <abhinav@fynarfin.in>