Switch to tabs in documentation to separate SBT and Mill instructions.

[Baccata]

See original comment from Jakub here : https://github.com/disneystreaming/smithy4s/pull/1455#discussion_r1551666855

suggestion: UI-wise I think we could improve upon these code snippets by splitting them in two, in addition we could use more advanced Docusaurus features such as mdx tabs. They're nice in the fact that you can make them synchronized, so that picking Mill once updates all the visible tab groups. Example: https://scala-php.org/docs/getting-started/ (sbt/mill tab groups are synced)