fix(docs): Update Growthbook Feature Flag integration docs

[ahdriel]

What does this PR do? What is the motivation?

#26999 was merged yesterday (Feb 10) but between the time it was created & merged the docs for Feature Flags was changed/consolidated on #25073 so it is not showing up on the live docs website.

So this updates it to use the new URL format & include the instructions on the new document.

Merge instructions

Merge readiness:

• Ready for merge

[rtrieu]

/merge

[dd-devflow]

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[rtrieu]

/merge

[dd-devflow]

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2025-02-19 20:15:23 UTC ℹ️ Start processing command /merge

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2025-02-19 20:15:28 UTC ℹ️ MergeQueue: waiting for PR to be ready

This merge request is not mergeable yet, because of pending checks/missing approvals. It will be added to the queue as soon as checks pass and/or get approvals.
Note: if you pushed new commits since the last approval, you may need additional approval.
You can remove it from the waiting list with /remove command.

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2025-02-19 20:50:26 UTC ℹ️ MergeQueue: This merge request was already merged

This pull request was merged directly.