AsyncAPI: Add help/{command} endpoint

Suggested by Marc DiPasquale in Slack comment Slack

We should create such an endpoint help/{command} to return instruction of given command to the user. For example help/generate should return available parameters such an available templates etc.

help/{command} is just a suggestion, we can go with other endpoint(s).

Also, what is very important, that endpoint should be done in a similar way as in our main CLI - GitHub - asyncapi/cli: CLI to work with your AsyncAPI files. You can validate them and in the future use a generator and even bootstrap a new file. Contributions are welcomed!, so main help endpoint should return all available endpoints and particular one like help/generate should return details about given command.

Learn more here asyncapi/server-api#144

Mentors:
@BOLT04

Project Repo:
tbd

Expected Difficulty:
Easy-Medium

Expected Time Commitment:
175 Hour