[Get started]: Linux install instructions

[garrytrinder]

We currently provide install instructions for Windows and macOS in the Get started section.

We should also provide instructions for Linux (Ubuntu).

[gavinbarron]

We probably need to ensure that we create separate guidance for WSL vs bare metal Linux too.

[garrytrinder]

Good shout @gavinbarron, I'll make a new issue to capture the WSL instructions.

[waldekmastykarz]

PR open, pending editorial review