Update checkpointing documentation to mark resume_from_checkpoint as deprecated (#20361)

[WangYue0000]

This PR updates the checkpointing documentation to mark resume_from_checkpoint as deprecated
and provides examples of the recommended ckpt_path method in the Trainer.fit() function.

Changes made:

• Added a warning box in the "Resume training state" section to discourage the use of resume_from_checkpoint.
• Updated examples to reflect the correct usage of ckpt_path.
• Ensured consistency across the documentation by replacing all occurrences of the deprecated parameter.

Fixes #20361.

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📚 Documentation preview 📚: https://pytorch-lightning--20477.org.readthedocs.build/en/20477/

[WangYue0000]

@Borda @lantiga Hi！Could you please take a look at this PR when you get a chance? It updates the checkpointing documentation to address #20361. Thanks, hope my solution helps!

[lantiga]

Thank you for your contribution! I added a couple of comments

[WangYue0000]

@lantiga Thank you for the detailed feedback! I've addressed all your suggestions, including:

• Updated terminology from "parameter" to "argument."
• Added version details for the deprecation.
• Moved the warning box to appear after the correct usage.
• Improved the phrasing as per your recommendation.

Please let me know if there are any additional changes needed!

[lantiga]

Looks great, thank you @WangYue0000 !