give a link to Vue 2 slots docs page for users who are coming from Vue <2.6.0

[aghArdeshir]

I personally had a hard-time finding exact changes needed to be done and I had to constantly switch tabs between Vue 2 slots page, migration guide to Vue 3, and Vue 3 docs page for more info. I belive leaving a link that informs the migration is a little different for Vue <2.6.0 users is helpful.

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[aghArdeshir]

rebased on this PR: https://github.com/vuejs/v3-migration-guide/pull/59 because pipelines were failing