fix: Replace placeholder installation URL with pip instructions

[Copilot]

Addresses review feedback on placeholder installation script URL (https://install.fsl-continuum.sh) that would fail for users attempting quick start.

Changes

• Replaced curl-based placeholder script with pip install fsl-continuum
• Added source installation alternative using pip install -e .
• Installation instructions now align with actual project structure defined in pyproject.toml

# Before (broken placeholder)
curl -sSL https://install.fsl-continuum.sh | bash

# After (working pip installation)
pip install fsl-continuum

# Or install from source
git clone https://github.com/your-org/fsl-continuum.git
cd fsl-continuum
pip install -e .

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