[dev-v5] Add migration guide for `FluentCombobox`

[MarvinKlein1508]

https://fluentui-blazor-v5.azurewebsites.net/Lists/Combobox#migrating-to-v5

[vnbaaij]

Add this document in a new PR. Look at the existing docs in examples/fluentui.demo/getstarted/migration for the style and structure we already use. Link to the new docs from the combobox docs page.