Added documentation for MaaS Extension (apache#561)

* Added documentation for MaaS Extension

* Some corrections

* doc changes

* template changes

* few edits

* Shorten the text

* Doc and image changes

* more details

* Update source/adminguide/extensions/inbuilt_extensions.rst

* Update source/adminguide/extensions/inbuilt_extensions.rst

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Co-authored-by: Rohit Yadav <[email protected]>

Author: harikrishna-patnala