Mention the casadi conda package in the docs

[alvgaona]

Current Situation

The documentation currently doesn't mention the availability of CasADi through conda-forge channels, which provides a convenient installation method for both Python and C++ users.

Proposed Addition

I suggest adding information about installing CasADi through conda-forge to the installation documentation. The conda-forge package offers several advantages:

• No need to build from source for C++ projects
• Avoid system-level installation
• Compatible with modern package managers (pixi, mamba)
• Simplified dependency management
• Cross-platform compatibility

Example Installation Commands

conda install -c conda-forge casadi
# or
mamba install -c conda-forge casadi
# or
pixi add casadi

This addition would make the documentation more complete and provide users with all available installation options.