Wrap OS-specific instructions in collapsible sections

The README is a long, linear document. Readers following the guide on a specific OS currently have to scroll past several screenfuls of instructions for other platforms to reach the next step. This is especially noticeable in the Install software section, where seven OS variants appear back-to-back.

Collapsing alternative OS blocks means:

Each reader sees only the instructions relevant to their platform — no scrolling past irrelevant content
The overall document length on screen is dramatically reduced, making the flow of the guide easier to follow
All platforms remain equally discoverable via the labelled `` toggles, so nothing is hidden or removed