doc: mention that multiple dvc.yaml files are supported

[jorgeorpinel]

Extracted from #1615 (comment)

Note needed probably in

• https://dvc.org/doc/user-guide/dvc-files-and-directories,
• https://dvc.org/doc/command-reference/run, and
• maybe https://dvc.org/doc/command-reference/repro

Anywhere else?

[skshetry]

Can we prioritize this? 🙂

[skshetry]

If it's not possible for now, I can work on this next week.

[shcheklein]

If it's not possible for now, I can work on this next week.

@skshetry that would be great! feel free to take it :)

[jorgeorpinel]

@skshetry hi did you ever get around to this? Thanks

UPDATE: Never mind I see this is only hinted casually in dvc run but I added some more notes as part of #2075

[jorgeorpinel]

Closed by #2075.

[skshetry]

Closed by f5978df

[jorgeorpinel]

I guess I forgot to actually click on "close" 😬

Ty