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 15610565253

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 15574750714:	-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.

[edoaltamura]

The main motivation is to make it clear in which format the io is done, and partially also staying consistent with the syntax used in other libs like pandas, where from_* methods generally denote classmethods. Also added the reno about this change.

[woodsp-ibm]

Also should not this tutorial 09_saving_and_loading_models.ipynb be updated as well to use the replacement methods not the deprecated save/load.