Docs: Enhance installation instructions

[IngwiePhoenix]

I fought with this for way too long... Wanted to use uv, but that did not work. So instead, use pipx:

# If you don't have it, install it
uv tool install pipx

# Install:
pipx install qwen-tts

# Clean up:
pipx runpip qwen-tts uninstall -y torch torchvision torchaudio

# CUDA-ify
## Change the `cu130` to your CUDA version
pipx inject qwen-tts torch torchvision torchaudio --index-url https://download.pytorch.org/whl/cu130

# Fix: pytz is not installed by default
pipx inject qwen-tts pytz

On Windows, you also need to winget install ChrisBagwell.SoX

I could not get fast-attn working, but it's alright.

Now, I would like to propose a request to re-shape the docs a little, to make installation and bringup of the demo a little more easy :)