Update Subkey installation tutorial

[ggwpez]

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Chrome

Bug report for the website

The subkey installation page mentions cloning the repo!? https://docs.substrate.io/reference/command-line-tools/subkey/

Instead of three steps it should either be (will need to be updated after the monorepo):
cargo install --git https://github.com/paritytech/substrate.git subkey or even better but not sure if we already release this properly: cargo install subkey

Would also close https://github.com/substrate-developer-hub/substrate-docs/issues/2021

Steps to reproduce the problem

No response