Add symlink to wai-InvolveUsersAll VTT metadata

[remibetin]

Context: https://github.com/w3c/wai-translations/issues/117

🚩Dependencies

Merge https://github.com/w3c/wai-InvolveUsersAll/pull/26 with this PR before publishing

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[remibetin]

Deprecated by new method: see https://github.com/w3c/wai-translations/issues/125