Indicate that Group Sequence Providers can use YAML

In the docs, it was unclear that you could use group sequence providers without using annotations. (It took me some trial and error to figure out how to do it.) I've indicated that it can also use YAML, and how to do so. I've never used XML for validation, so I don't feel qualified to add info about XML to the docs.
minor #3369 Indicate that Group Sequence Providers can use YAML (karptonite)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3369).

Discussion
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Indicate that Group Sequence Providers can use YAML

In the docs, it was unclear that you could use group sequence providers without using annotations. (It took me some trial and error to figure out how to do it.) I've indicated that it can also use YAML, and how to do so. I've never used XML for validation, so I don't feel qualified to add info about XML to the docs.

Commits
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7512b65 Indicate that Group Sequence Providers can use YAML