Document OIDC support for SinkBinding & ContainerSource

[creydr]

Currently we don't document the OIDC support of the SinkBinding and ContainerSource in https://knative.dev/docs/eventing/features/sender-identity/ (sender-identity.md).

We should document that, as soon as the SinkBindings/ContainerSource sink has an audience and OIDC is enabled, a file will be mounted to the SinkBinding/Container under /oidc/token containing the OIDC token with the audience for the sink

[chaliuu]

/assign @chaliuu