Document Copy Task support for copying a directory

Document a new feature of Copy Task from PR #8843, concerning support for copying a directory.
See conversations from #5881.

I suggest to document it in the docs for the [Copy Task](https://learn.microsoft.com/en-us/visualstudio/msbuild/copy-task?view=vs-2022)

Start with PR #8843 and the conversations in issue #5881 to understand the directory-copying behavior, then update the linked Copy Task documentation page. Done means the page documents the new directory-copying support accurately and is consistent with the feature described in the PR.

Written by the indexing model from the issue text.