Docs example uses `mv.average()` but `MgVideo` has no `average` method

[alexarje]

• Reproduce issue average() not working/available #325 using the code snippet from docs/examples.md (Example 8).
• Determine whether:
  • average() was renamed/removed (update docs to correct method), or
  • it should exist on MgVideo (implement/restore and add tests).
• Acceptance criteria:
  • Example 8 in docs/examples.md works as written or is updated to the correct API.
  • Add a small regression test (or minimal smoke test) covering the documented behavior.