Improve the qiskit.algorithms docs: clearly show legacy vs. primitive algorithms

[Cryoris]

Emphasize the split in primitive-based and legacy algorithms more clearly

Summary

More clearly emphasize that for the (minimum) eigensolvers there is currently a duplication of algorithms: the legacy, QuantumInstance-based algorithms that are pending deprecation and the new, primitive-based ones.

Details and comments

An alternative would be to keep the separate headings and link to the non-deprecated one, like in the screenshot for the time evolvers below. However since the headings are "Eigensolvers" vs. "Eigen Solvers" I think it would be nice to merge them.

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[coveralls]

Pull Request Test Coverage Report for Build 3984526255

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 84.89%

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Totals
Change from base Build 3971839797:	-0.03%
Covered Lines:	66517
Relevant Lines:	78357

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💛 - Coveralls

[ElePT]

LGTM! I will wait to see the rendered docs before merging, just in case.

[mtreinish]

I've marked this on hold to preserve the CI bandwidth. We can merge this after rc1 is tagged since it's a bugfix release. After rc1 is tagged lets remove the on hold label

[woodsp-ibm]

In the future, primitive-based Minimum Eigensolvers should be used in place of the legacy :class:.QuantumInstance-based ones.

Ideally we would like people to start using these new ones now (not in the future) for any new code (and maybe even update existing code), as the the legacy ones ones, while they continue to work, will soon be deprecated and later removed. Hence the "In the future, " bit seems like it could be improved (I just quoted one example above). In the future you will only be able to use these. Now you can too and indeed we recommend you do use these rather than the legacy versions.

[Cryoris]

That's a good point, how about something like

These algorithms are based on the Qiskit Primitives, a new execution paradigm that replaces the use
of :class:`.QuantumInstance` in algorithms. To ensure continued support and development, we recommend 
using the primitive-based Eigensolvers in place of the legacy :class:`.QuantumInstance`-based ones.

?

[woodsp-ibm]

That's a good point, how about something like

Looks much better to me thx!

[Cryoris]

Done in bf1f6cc 👍🏻