docs: mention /flagz for feature gate verification

[intojhanurag]

Description

This PR updates the Enable or Disable Feature Gates documentation to mention the
/flagz debugging endpoint as an additional way to verify feature gate configuration.
It adds a short section explaining that /flagz exposes the command-line flags used
to start a component and links to the Kubernetes z-pages documentation for details.

Issue

Closes: #53580

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[intojhanurag]

Hey @lmktfy , Please take look on it . If you have a moment.

[lmktfy]

This will change within a fortnight (due to to v1.35 release).

The simple thing to do is to omit the entire note. After v1.35 is released we could add an updated note.