[cover-image] Deprecation guide once Payload's native video cover covers all use-cases

[tsemachh]

Plugin: cover-image · Priority: P2 · Tracked by ROADMAP.md

Problem

Some use-cases now covered by Payload natively.

Proposed solution

Add a deprecation/migration guide pointing to Payload's native video cover where applicable.

Acceptance criteria

• Guide published.
• Linked from README.
• Lists what's still unique to this plugin.

How to claim

Comment on this issue (e.g. "I'd like to work on this") and a maintainer will assign it to you. Open a PR referencing this issue (Closes #<n>). Discuss design questions here or on the Payload Discord #plugins channel.