Update serialising methods and instructions

[edoaltamura]

Summary

Mark the legacy save/load methods as deprecated (v0.9.0) and point users to to_dill/from_dill.

Add short usage examples under each serialisation method.

Clarify raised exceptions in argument docs.

Details and comments

Closes #891

[coveralls]

Pull Request Test Coverage Report for Build 15541719624

Details

• 6 of 10 (60.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.07%) to 90.8%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_machine_learning/algorithms/serializable_model.py	6	10	60.0%

Totals
Change from base Build 15535750625:	-0.07%
Covered Lines:	4530
Relevant Lines:	4989

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💛 - Coveralls

[woodsp-ibm]

• Needs a reno release note to cover the deprecation of save/load and these new replacement methods.

I get that the improved example may well go towards solving the referenced issue. Its just less clear to me how changing the names of the methods helps in that regard. I guess if one cares it makes it clear that its saved as a dill file, otherwise, while names can be changed maybe I am missing something.