Give `public_baseurl` a default value (#9159)

changelog.d/9159.feature

docs/sample_config.yaml

synapse/api/urls.py

synapse/config/_base.py

synapse/config/emailconfig.py

synapse/config/oidc_config.py

synapse/config/registration.py

synapse/config/saml2_config.py

synapse/config/server.py

synapse/config/sso.py

synapse/handlers/identity.py

synapse/rest/well_known.py

tests/rest/test_well_known.py

tests/utils.py