Add ghcr.io migration banner to README.md

[twoGiants]

Changes

Add ghcr.io migration banner to README.md. Add types to code blocks in and update links and references in manifests to use ghcr.

Issue tektoncd/website#645.

/kind documentation

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[tekton-robot]

Hi @twoGiants. Thanks for your PR.

I'm waiting for a tektoncd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[vinamra28]

/lgtm
/ok-to-test

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vinamra28

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