Refac generated documentation

Closes #183 (closed) and fixed the xtask gen-doc that was not compiling anymore.

cargo xtask gen-doc will now generate the rust autogenerated documentation (the full (and bloated) documentation is necessary for Substrate pallets calls). The docs/api/runtime-calls.md will be generated with reference to the rust autogenerated documentation based on the BASE_URL variable.

The only thing left to do is to decide where to store the rust autogenerated documentation. Then, a CI with the necessary server permissions can simply run:

BASE_URL="https://url/to/endpoint" cargo xtask gen-doc
rsync -a ./docs/api/doc user@server:/path/to/endpoint/