Add optional uv installation instructions to README & make uv command visible on Landing Page next to pip install window

[datasound-projects]

What this changes

• Adds an optional uv install command to the README
• Adds the optional uv extras install example uv pip install "quantjourney-bt" or "quantjourney-bt[data,wf]"
• Keeps the existing pip instructions unchanged

Why

• uv gives users a fast modern Python install option
• It works well with this package because quantjourney-bt already uses standard pyproject.toml packaging
• It is helpful for users and contributors who already use uv

Benefits of uv

• much faster installs
• faster and more reliable dependency resolution
• quick virtual environment creation
• cleaner all-in-one Python workflow
• efficient caching across projects
• useful for faster local setup and CI runs

Extra request

• Please also mirror this on the website landing page next to the current pip install window, so users can see the optional uv install path too