Improve references to workflow definition files

[jverce]

What was changed

• Update extension of referenced files to match the currently existing ones
• Transform the file references into an enumerated list with links to the corresponding files

Why?

While reading these docs, I had a hard time understanding the references to the workflow files that were made in the introduction section. I spent some time going through the repo until I found those files, and I'd like future readers to have an easier time.

Checklist

3. Any docs updates needed?

• src/main/java/io/temporal/samples/dsl/README.md

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[jverce]

@tsurdilo does this change make sense? Thanks!

[tsurdilo]

@jverce thanks for the pr!