[docs] Polish migration guide

[oliviertassinari]

Small content formating issues I could spot on the page.

[mui-bot]

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• docs/data/material/migration/migrating-from-deprecated-apis/migrating-from-deprecated-apis.md
• docs/data/material/migration/migrating-to-v6/migrating-to-pigment-css.md
• docs/data/material/migration/migrating-to-v6/migrating-to-v6.md
• docs/data/system/migration/migrating-to-v6/migrating-to-v6.md

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