feat(doc): Add note about default pin changes to migration guide

[lucasssvaz]

Description of Change

Add note to the migration guide about default pin changes for UART. Also fix some formatting issues.

[github-actions]

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📖	🎉 Good Job! All checks are passing!

👋 Hello lucasssvaz, we appreciate your contribution to this project!

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[SuGlider]

LGTM

[lucasssvaz]

@SuGlider Let's wait #9515 be merged first before putting the Pending Merge tag. This way the bot won't be crazy with unrelated changes 😆