OIDC doc: adds missing jwt_config values to authentik example

[maxkratz]

This PR addresses #17896 by extending the Synapse documentation regarding the necessary jwt_config entry in the example on how to connect authentik via OIDC to Synapse.

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[maxkratz]

fyi: I've also created a PR on the authentik repository to update their documentation as well: goauthentik/authentik#16860

[anoadragon453]

Thank you for doing this so long after addressing the problem in your local setup. Other users will appreciate it!