Add links for ngspice or other circuit simulators.

[landerich]

Problem Description
Instructions are not linked to a circuit simulator software page or installation guide.

Expected behavior
Expect to see more links for everything that is needed regarding SPICE simulations and other dependencies required to run CharLib.

To Reproduce
Provide detailed steps to reproduce the behavior. Include your yaml configuration and spice files if relevant.

You can use triple backticks ``` above and below code to create a nicely formatted code block. Please use this feature if you include yaml or spice.

System Configuration

• OS: Windows 11
• Python Version: 3.13.6
• CharLib Version: 2.0.0
• PySpice Version: 1.6
• Spice Simulator: -missing
• Spice Simulator Version: -missing

Additional context
Overall this is a issue request to fix instructions issues or ambiguous descriptions.

[infinitymdm]

I think this issue is a duplicate of #81. If that's not what you intended, feel free to reopen and add additional context.