Document log(g f) cutoff option in TARDISAtomData (#273)

[manas-dhyani]

Documentation Update

This PR updates the Quickstart guide to document the log(g f) cutoff used in Carsus.

• Default cutoff: -3
• Users can modify it via "lines_loggf_threshold" inside levels_lines_param in TARDISAtomData.
• Added an example demonstrating how to change the cutoff.
• Explained how modifying this value affects the included spectral lines.
• Fix: Corrected the broken "Notation in Carsus" link.

📌 Resources

• Carsus Quickstart Guide

🚦 Testing

• No testing required as this is a documentation change.

☑️ Checklist

• I updated the documentation according to my changes.
• I built the documentation by applying the build_docs label.
• I requested two reviewers for this pull request.

Closes #273.

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