NLB-2234: update vmss backend documentation

[Naveen-Gopu-F5]

Proposed changes

• Included the steps/documentation about the vmss backend which helps in dynamic update of upstreams in NGINXaaS

Checklist

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• I read the Contributing guidelines
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Footnotes

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[github-actions]

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[arussellf5]

This is excellent documentation! Thank you

[JTorreG]

wait for a 2nd writer approval before merge. This PR has new docs.

[mjang]

I think the big question for me in this PR is "What's a 'dynamic upstream'"

[Naveen-Gopu-F5]

I think the big question for me in this PR is "What's a 'dynamic upstream'"

We are upstreams concept in Nginx Configurations . It is block used to define the backend servers. Dynamic upstream mean we can configure the backend servers dynamically without the users manual intervention.