Problem: Missing support of uploading cpp documents to gitbook platform (fix #396)

[damoncro]

Close:

• Problem: Missing support of uploading cpp documents to gitbook platform #396

Solution:

• Add cronos-play-docs as submodule
• Update Makefile cpp-docs and cpp-cronos-play-docs

[damoncro]

Sobumodule permission problem happens in CI, probably it is a private repo must clone by git (still need to check)? But it is a way to generate docs from defi-wallet-core by make cpp-docs, the artifacts will directly go into the submodule cronos-play-docs repo.

[tomtau]

as discussed on Slack, this can be done on https://github.com/crypto-com/play-cpp-sdk