Expand and improve workers documentation and example configuration

This issue has been migrated from [#9072](https://github.com/matrix-org/synapse/issues/9072).

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A general call;
- Add more documentation surrounding workers (just in general)
- [ ] Add example configuration file (a-la `sample_config.yaml`) for each worker type, or for all workers in one
- [ ] Add docker-compose file that includes ready-to-work example for synapse workers

Imo: Currently i think workers are very hard to set up or figure out on its own, because knowledge of it is mostly limited to large setups, and thus that knowledge is never reflected in documentation, or workers seen as an asset to be documented with the same priority.