Document how to deploy plugins

This issue has been migrated from [#8944](https://github.com/matrix-org/synapse/issues/8944).

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We have plugin support used in various places, some of these are documented to a greater or lesser extent (e.g [SSO mapping providers](https://github.com/matrix-org/synapse/blob/develop/docs/sso_mapping_providers.md)), but we should have a generic doc they can all point to explaining:

1. ~How to write a plugin, e.g the function signatures of `__init__` and `parse_config`,~ (see [docs](https://matrix-org.github.io/synapse/latest/modules/writing_a_module.html))
2. How to deploy these plugins in practice (i.e. make them a python package and install them in the venv?)