I am trying to use HexRay's decompiler API, however having trouble due to its lack of documentation. Is there any means so That I can get to see the Python implementation of decompiler API?
As far as I remember IDA license allows reverse engineering of IDA itself. If suggestion that same rule is applicable to the decompiler is correct - the most approachable way to see the implementation is to reverse engineer the plugin - despite the fact that at least part of it should be significantly obfuscated.
There is, however, additional source of the information you can use, the documentation about C++ decompiler SDK is opened and can be found here.
It includes the source of hexrays.hpp file (here) with decent amount of comments that may be helpful.