[Feature] [Doc] Add extension doc for fluxcd/flagger

[wu-sheng]

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• [X] I had searched in the issues and found no similar feature requirement.

Description

We have an ongoing https://github.com/fluxcd/flagger/pull/1273 for skywalking integrating with flagger. Once it is done, we should add a doc under Extension menu to explain how to use it, and SkyWalknig's role in this integration.

This doc should similar with what we need to write at flagger side.

Use case

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Related issues

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Are you willing to submit a PR?

• [ ] Yes I am willing to submit a PR!

Code of Conduct

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