Add Migration guide from BackendV1 to BackendV2

[ElePT]

This PR addresses #2704. As described in the issue, this was part of the API reference of Qiskit. Given the extended support period of 1.4, which still supports BackendV1, I think it would make sense to keep this content alive a bit longer than the BackendV1 interface to help users transition to the 2.0 standard.

This migration guide can also be linked in the general 2.0 migration guide.

Notes

I have copy-pasted the original content and reformatted to mdx, but I also want to update the text a bit to make it more guide-y. This is why the RP is currently WIP.

preview: https://qiskit.github.io/documentation/pr-2705/migration-guides/qiskit-backendv1-to-v2

[beckykd]

I'm putting the new migration guide in the TOC to remove the lint error. Do you think it makes sense to put it in a section for Qiskit 2.0? @kaelynj is working on that guide.

[Eric-Arellano]

I'm putting the new migration guide in the TOC to remove the lint error. Do you think it makes sense to put it in a section for Qiskit 2.0? @kaelynj is working on that guide.

I think Elena's suggestion in the original post is great: have a dedicated guide because this is niche, but link to it from the general 2.0 guide. Becky, I like how you set up the ToC

[beckykd]

FYI, @ElePT links to APIs automatically go to the newest release (1.4), so actually putting 1.4 in the link breaks it unless 1.4 is not the current release :)

[Eric-Arellano]

links to APIs automatically go to the newest release (1.4), so actually putting 1.4 in the link breaks it unless 1.4 is not the current release :)

Generally, yes. But these links are for APIs that are going to be deleted so it's good that Elena is putting 1.4.

[beckykd]

links to APIs automatically go to the newest release (1.4), so actually putting 1.4 in the link breaks it unless 1.4 is not the current release :)

Generally, yes. But these links are for APIs that are going to be deleted so it's good that Elena is putting 1.4.

I got lint errors on the links with 1.4 in them. Should I have left them?

[beckykd]

@ElePT, I added a second version of the table to try to make it skinnier, so the Notes column shows up. That outlier "instructionDurations" messed it up though, so I pulled that into a different table. Thoughts? I left the original table in there too, with a note to scroll for Notes. I also tried to reorganize it to make it easier to follow.

[ElePT]

Thanks for the edit @beckykd! I really like the warning to scroll for notes, but I am not sure about the "skinnier" table. These are my thoughts:

1. I feel like seeing backend.whatever() is a bit easier to contextualize and copy-paste than just whatever() or .whatever()
2. In this table we are mixing a bit methods and attributes. For example: backend.configuration().coupling_map is calling a method and then getting an attribute from the backend configuration class, the alternative for V2 is backend.coupling_map which is retrieving an attribute directly (no method). So having titles that say "BackendV1 method" isn't the most accurate.
3. I would be ok with the other 2 points if the change allowed the table to fit in the page without scrolling, but I think it doesn't, so maybe it's not so bad to have it be a bit wider and keep the original?

[jyu00]

One small comment but the rest looks good.

[beckykd]

Approving this, but it can't be merged until the infra team fixes the issue with the 1.4 links.

[arnaucasau]

Thanks for the PR! I merged main to fix the link checker 👍