Move platform selection to runners "register" step

🛑 This work is blocked until the GCP runner work is merged (see this epic for more info).

Background

We have two pages during our for our registration process, the "Create Runner" page and the "Register Runner" page.

Currently, users are prompted to select the platform they want in the first step, but this information is not strictly necessary to create runner (or it's token).

Description

I propose moving the platforms to the second step in the register page.

before	after

Advantages

1. The information of the platform selected is more relevant in the registration step, users can decide which platform they will use, and the instructions can adapt.

2. Installation instructions can adapt to the platform selection:

Disadvantages

• Kubernetes and Docker instructions workflows are different, so users may need some other setup steps even before they create the token (I'd like to confirm this one though).