Documentation of the Third Party Event Rules

This issue has been migrated from [#7954](https://github.com/matrix-org/synapse/issues/7954).

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There's currently no documentation in `docs/` on Synapse's Third Party Event Rules. Ideally the documentation would have:

* An explanation of what it is and why it exists (the sample config [does an ok job](https://github.com/matrix-org/synapse/blob/b975fa2e9952f1f8ac2cddb15c287768bf9b0b4e/docs/sample_config.yaml#L2350-L2360))
* How to configure it (again, sample config)
* What methods are available to be overridden, and which are optional and required (one can currently figure this out by looking at [the class itself](https://github.com/matrix-org/synapse/blob/develop/synapse/events/third_party_rules.py#L1)
* Any examples of implementations (I believe [DINUM's one](https://github.com/matrix-org/synapse-dinsic/tree/dinsic/synapse/third_party_rules) is currently the only known example)

All following a similar format to the [Password Auth Providers documentation](https://github.com/matrix-org/synapse/blob/develop/docs/password_auth_providers.md).