for _config and _toc files, only `yml` extension is recognized not `yaml`

[Connor Stone, PhD]

Describe the bug

context
When I do ___.

expectation
I expected both yaml and yml to be recognized since they are both standard convention.

bug
But instead only yml is recognized.

problem
This is a problem because I lost a whole day finding this.

Another possibility would be to throw a warning whenever there is no _config.yml found.

Reproduce the bug

try making a _config.yaml file, it wont work and no indication of a problem is given.

List your environment

No response

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