Windows uv venv activation

The README installation section
```bash
uv venv --python 3.12 && source .venv/bin/activate
```

Two issues with this
- the `&&` was confusing for some users, so break it down to two commands
- workshop participants with Windows OS were confused because this doesn't work on their machines

Update the Installation section in README.md to cover both shells. For example, using tabbed/OS-split instructions:
``` bash
# macOS / Linux:
source .venv/bin/activate

# Windows Powershell
.venv\Scripts\Activate.ps1

# Windows Git Bash
source .venv/Scripts/activate
```