docs: Add example of template-level volume declaration.

[danxmoran]

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I have written unit and/or e2e tests for my change. PRs without these are unlike to be merged.
• Optional. I've added My organization is added to the USERS.md.
• I've signed the CLA and required builds are green.

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This is a feature that my team now makes heavy use of, but we had to read through the workflow_types.go file to realize it was even possible.

[codecov]

Codecov Report

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@@            Coverage Diff            @@
##             master    #2542   +/-   ##
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  Coverage          ?   11.14%           
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  Files             ?       75           
  Lines             ?    31739           
  Branches          ?        0           
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  Hits              ?     3538           
  Misses            ?    27720           
  Partials          ?      481

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[simster7]

Thank you!