add error mitigation explanation notebook

[kevinsung]

Fixes #1355 .

No material on PEA yet because it's not exposed as an option.

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[kevinsung]

Hmm the embedded images seem to be giving the CI issue. Gonna need to investigate...

[javabster]

Looking really good! I think this is going to be a very helpful docs page.

I left some in line comments and I have these general ones:

• please add a "next steps" section like we do for the other docs pages. I think we can add a link to the tutorial on combining EM techniques, as well as maybe the API reference
• Do you think we should add a section about PEA? I know it's technically only available through the experimental branch for now but we have docs for other experimental features so I can see a case for including here too

[kevinsung]

@javabster I've addressed your comments.

Do you think we should add a section about PEA? I know it's technically only available through the experimental branch for now but we have docs for other experimental features so I can see a case for including here too

I think we should document PEA once it's generally available, since the way that the options are exposed would probably affect the exposition. We shouldn't document options that aren't available yet.

[abbycross]

I caught a few small word things - otherwise it reads very well, as always!