Add documentation for signal folder/factory

[eakmanrq]

Slack context: https://tobiko-data.slack.com/archives/C07HJH22U9Z/p1725024070460339?thread_ts=1725006361.257769&cid=C07HJH22U9Z

Links from convo: https://github.com/TobikoData/sqlmesh/blob/main/sqlmesh/core/scheduler.py#L85-L100 https://github.com/TobikoData/sqlmesh/blob/main/sqlmesh/core/scheduler.py#L53-L77

Text:

Basically, create a file signals/__init__.py and create a function decorated with @signal_factory there. That function should return instances of the Signal class.