Update instructions to get rid of depreciated trusted_third_party_id_servers value

[q-wertz]

In some of the instructions (e.g. https://github.com/ma1uta/ma1sd/blob/master/docs/getting-started.md) there are still some places where the user is told to set the variable trusted_third_party_id_servers in homeserver.yaml.
This variable is depreciated with synapse v1.4.0 (see Changelog and PR #5875) and the instructions should be updated.

[ma1uta]

Thanks.

[q-wertz]

Closed with 90b2b53