Add an example of scrolling in CDK Drag and Drop

[cutiepoka]

This is to fix the issue that many people miss that you need an extra bit for scrolling to work with the CDK drag and drop.
#31519, #29161, and maybe this one #24553. There were a few others, but I cant find them now.

This adds documentation that shows you need to include the CdkScrollable otherwise scrolling wont work.

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[crisbeto]

The change looks good, but the PR is missing a CLA and there are CI failures.

[cutiepoka]

I think I fixed them. It should be good now.

[cutiepoka]

@mmalerba Did I mess something up?

I dont remember adding tests.

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