Create a machine-readable document for routing requests to workers

This issue has been migrated from [#12139](https://github.com/matrix-org/synapse/issues/12139).

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Constructing a reverse-proxy configuration for routing requests to workers is currently left largely as an exercise for the reader in https://matrix-org.github.io/synapse/develop/workers.html.

I think we should maintain a single source of truth which documents a mapping between endpoints and workers, which could ultimately be used as an input to other scripts which construct configuration files suitable for various reverse-proxies.

Start with the worker routing guidance in https://matrix-org.github.io/synapse/develop/workers.html and review how endpoint-to-worker mappings are currently described. Done means there is a maintained machine-readable source of truth that documents the mapping and can serve as input for reverse-proxy configuration generation.

Written by the indexing model from the issue text.