Include JSON-RPC Specs in paritytech.github.io

[kianenigma]

Similar idea as #5, but for RPC.

For example, the best place now to go and see the list of Substrate RPCs and their return type is https://polkadot.js.org/docs/substrate/rpc/, which makes little sense 🤦‍♂️.

Could be part of rust-docs.

[kianenigma]

https://paritytech.github.io/json-rpc-interface-spec/ already exists, so the only action item here is to update https://paritytech.github.io/ to show it.

https://github.com/paritytech/json-rpc-interface-spec

[kianenigma]

This is now done.