NGINXaaS Platform metric migration

[arpith-f5]

Proposed changes

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following format:

Problem: Add instructions for customer to migrate from custom metric to platform metrics for NGINXaaS

Solution: created a new page with instructions.

Testing: Local testing.

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specific feedback, add them here.

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Closes #ISSUE

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[arpith-f5]

I have hereby read the F5 CLA and agree to its terms

[amudukutore]

I'm wondering if "Getting Started" is the right section for this. Maybe this is better to have under Logging and monitoring ?

Note: I have a similar concern with Migrate from Standard to Standard V2 but that is already where it is so we have to live with it until all Standard deployments have been migrated.