Update top level provider documentation for BackendV2

[mtreinish]

Summary

As a follow-on to #5885 this commit updates the top level
qiskit.providers module documentation for BackendV2. This primarily
consists of two things, updating the writing a provider section to use
BackendV2 instead of BackendV1 and adding a migration section on user
facing changes between BackendV1 and BackendV2.

Details and comments

[mtreinish]

This PR is based on #5885's branch and is on hold until #5885 merges. After #5885 merges I'll rebase this on main and remove the on hold label. To see the changes in this PR you can look at the last commit: 4e4b4d1

[coveralls]

Pull Request Test Coverage Report for Build 1527872187

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 82.963%

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Totals
Change from base Build 1527703798:	0.008%
Covered Lines:	50767
Relevant Lines:	61192

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💛 - Coveralls

[jakelishman]

Just a quick read through. I don't know enough about what's happened in the main BackendV2 PR to properly review the content, but the bits I do know looked correct to me.

[mtreinish]

Ok, now that #5885 has merged I've rebased this on the merged version and it's no longer blocked

[1ucian0]

The following sentence sounds repetitive:

For a simple example of a provider the
`qiskit-aqt-provider <https://github.com/qiskit-community/qiskit-aqt-provider>`__
package is worth using as an example.

I suggest:

For a simple example of a provider, see the
`qiskit-aqt-provider <https://github.com/qiskit-community/qiskit-aqt-provider>`__
package.

[1ucian0]

Is this spacing intentional?

Suggested change

	:meth:`~~qiskit.providers.BackendV2.run` method.
	* Add any custom gates for the backend's basis to the session
	:class:`~qiskit.circuit.EquivalenceLibrary` instance.
	:meth:`~~qiskit.providers.BackendV2.run` method.
	* Add any custom gates for the backend's basis to the session
	:class:`~qiskit.circuit.EquivalenceLibrary` instance.

[mtreinish]

It is, the custom gates thing is a sub-bullet of building a backend (as it's an optionally a part of building a backend class) see where @jakelishman asked the same thing: #7323 (comment)

[mtreinish]

The following sentence sounds repetitive:

For a simple example of a provider the
`qiskit-aqt-provider <https://github.com/qiskit-community/qiskit-aqt-provider>`__
package is worth using as an example.

I suggest:

For a simple example of a provider, see the
`qiskit-aqt-provider <https://github.com/qiskit-community/qiskit-aqt-provider>`__
package.

Done in: 254270d I also updated the link to the repo's current home

[1ucian0]

Thanks!