Add documentation for extension of volume policy feature

[shubham-pampattiwar]

Thank you for contributing to Velero!

Please add a summary of your change

Add documentation for extension of volume policy feature

Does your change fix a particular issue?

Fixes https://github.com/vmware-tanzu/velero/issues/7773

Please indicate you've done the following:

• [x] Accepted the DCO. Commits without the DCO will delay acceptance.
• [x] Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• [ ] Updated the corresponding documentation in site/content/docs/main.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.67%. Comparing base (4d48273) to head (2706667). Report is 39 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7779 +/- ## ========================================== + Coverage 58.64% 58.67% +0.03% ========================================== Files 344 345 +1 Lines 28720 28743 +23 ========================================== + Hits 16843 16866 +23 - Misses 10447 10448 +1 + Partials 1430 1429 -1 ```

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