v6 docs devx

[niklasp]

As a ink! newbie i tried working with v6 and noticed several things:

• https://use.ink/docs/v6/ the fast track is nice but a link to the getting started seciton would be great https://use.ink/docs/v6/getting-started/setup for a more detailed view
• i had a old version of cargo that was missing features required by ink6, tell user to rustup update stable if they run into problems or just run it anyway?
• the cargo install cargo-contract --version ^6 --lockederrors

no@m21:~/code|⇒  cargo install cargo-contract --version ^6 --locked
    Updating crates.io index
error: could not find `cargo-contract` in registry `crates-io` with version `^6`

have to use the cargo install --locked --force --git https://github.com/use-ink/cargo-contract command from the fast track

[Daanvdplas]

Check out #439