docs: document existing ACR image deployment (#6967)

[sagarkt]

Description

Documents how to use an existing Azure Container Registry in both supported scenarios:

• manually trigger a Devtron build and push the resulting image to ACR
• register and manually deploy an image that already exists in ACR through External CI

The guide covers cluster pull access, imagePullSecrets, manual deployment behavior, and the External CI webhook request. It also corrects the existing webhook example to use the backend's api-token header.

Fixes #6967

How Has This Been Tested?

• git diff --cached --check
• Verified all added relative Markdown links and anchors resolve
• Verified the documented api-token header against both backend authentication call sites

Checklist:

• The title of the PR states what changed and the related issue number.
• This PR contains the required documentation updates.
• I have performed a self-review of the changes.

Does this PR introduce a user-facing change?

Documentation only; no runtime behavior changes.

[bito-code-review]

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