Docs: add known limitation and workaround for multiple Git SSH creden…

[ab-ghosh]

Changes

• Updated docs/auth.md to document a known limitation when using multiple tekton.dev/git-* annotations.
• Added a new section describing the issue where only the first annotated Git SSH credential is used successfully.
• Documented a customer-verified workaround using a mounted workspace with a Secret that provides multiple SSH keys and a custom .ssh/config.
• Included an example Secret manifest to illustrate the workaround.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• pre-commit Passed
• Follows the commit message standard
• Meets the Tekton contributor standards (including functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings). See some examples of good release notes.
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

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[waveywaves]

/ok-to-test