docs: simplify version table

[Borda]

What does this PR do?

reduce text and make easier navigation

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📚 Documentation preview 📚: https://pytorch-lightning--21123.org.readthedocs.build/en/21123/

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 pytorch_lightning: Docs

Check ID	Status
docs-make (pytorch, doctest)	success	✅
docs-make (pytorch, html)	success	✅

These checks are required after the changes to docs/source-pytorch/versioning.rst.

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Thank you for your contribution! 💜

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[Copilot]

Pull Request Overview

This PR simplifies the version compatibility table in the PyTorch Lightning documentation by introducing a cleaner notation system and adding explanatory notes for better user navigation.

• Replaces verbose "(last tested X.Y)" notation with a more concise "◇X.Y" symbol
• Adds explanatory notes defining the meaning of "≥X.Y" and "◇X.Y" notations
• Maintains all existing version information while improving readability

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