Add the node-agent ConfigMap document.

[blackpiglet]

Thank you for contributing to Velero!

Please add a summary of your change

Does your change fix a particular issue?

Fixes #9122

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.12%. Comparing base (6a03071) to head (a7d7cc9).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9434      +/-   ##
==========================================
- Coverage   60.13%   60.12%   -0.01%     
==========================================
  Files         386      386              
  Lines       35865    35865              
==========================================
- Hits        21567    21565       -2     
- Misses      12717    12718       +1     
- Partials     1581     1582       +1     

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[blackpiglet]

Not sure why this PR didn't trigger website preview build actions.
I also created a cherry-pick commit in PR #9195. The modification was verified in that PR's actions.

https://deploy-preview-9195--velero.netlify.app/