Spellcheck API docs

We currently ignore API docs:

https://github.com/Qiskit/documentation/blob/be5918995fa62cfef8f6ad6ee5b92553b7c5faae/cSpell.json#L278-L283

While we can't easily control them*, they are still important parts of the documentation! We should aim to have no spelling issues. Some thoughts on how to approach this:

• Should we add cspell to the source repositories? That's useful so that they detect the problems more quickly. It is annoying to add to their workflow and have to duplicate cSpell.json, though, to allow-list words.
• We need an ignore mechanism for API docs. Ideally we can do that without changing cSpell.json; it doesn't work to add inline comments to the markdown. I'm wondering if we can instead ignore via our custom script.
• When adding, we should keep the default npm run check:spelling to ignore API docs, per the spirit of https://github.com/Qiskit/documentation/pull/792.

*Note that we can make changes to historical API docs safely, if we're confident we won't add new versions of the API docs from CI. That is, if we're confident they're the final docs for that release series.

Read cSpell.json around lines 278-283 and the custom spelling-check script, then run npm run check:spelling to understand the current behavior. Compare the proposed source-repository and API-doc handling approaches; done means API docs can be checked without breaking the default command's intended exclusions.