update cli version to 0.8 in docs

[panmona]

While doing the first time set-up I noticed that the getting started docs are not up-to date with the current CLI version so I updated all npm install commands to 0.8.

Also I noticed that the migration guide doesn't include guidance on 0.7 to 0.8 yet. Maybe you can add those?

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[panmona]

@googlebot I signed it!

[patrickhulce]

thanks very much @panmau!