Migration guide section on key attribute changes

[AjaiKN]

Description of Problem

This is my attempt to fix #379 (a couple of undocumented breaking changes in Vue 3 related to the key attribute).

Proposed Solution

I think this is technically two separate breaking changes:

• If you manually provide keys on v-if branches, then each branch must use a unique key.
• <template v-for> key should be placed on the <template> tag (rather than on its children).

However, I think it's probably best to keep them on the same page because they're related and often appear at the same time. For example, they appear together in this relatively common situation:

<!-- Vue 2.x -->
<template v-for="item in list">
 <div v-if="item.isVisible" :key="item.id">...</div>
 <span v-else :key="item.id">...</span>
</template>

<!-- Vue 3.x -->
<template v-for="item in list" :key="item.id">
 <div v-if="item.isVisible">...</div>
 <span v-else>...</span>
</template>

Additional Information

In my opinion, keys are one of the more confusing parts of Vue. That's why I included a "Background" section near the top that explains what the key attribute is. I'm not really sure whether this section should be there or not, so I'm totally fine with removing the "Background" if people think it's unnecessary.

[NataliaTepluhina]

Thank you @Aurelius333! This is a great addition to migration guide 👍🏻