Include `cds add approuter` in MT guide again

[tim-sh]

Adding App Router is mentioned explicitly, whereas the Cloud Portal facet that would include it is considered optional.

cf. this comment on the original commit.

These changes concern the Multitenancy Guide, featuring an excerpt from the Deployment Guide.

The Deployment Guide, in turn, also mentions adding App Router as a separate item, followed by Cloud Portal as only one of two options.