[Documentation]: Update for Stream and v4 in general

[naivary]

What version of Badger is the target?

v4

Documentation.

Some APIs changed for v4 and none of them are documented. For exmaple streaming API changed and its really hard to figure out how to work with the new API.

Additional information.

No response

[baryluk]

Would be nice to update at least https://dgraph.io/docs/badger/get-started/ to use v4 imports.

[baryluk]

Also in few places in docs, it says that cgo is required for zstd, which is no longer true.

[github-actions[bot]]

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