Adds docs for the AI features errors

[Jordi-PV]

Summary

This PR documents the errors that the Yoast AI features display.

Relevant technical choices

Technical choices that affect more than this issue:

None.

Test instructions

This PR can be acceptance tested by following these steps:

1. Open up https://document-ai-error-messages.yoast-developer.pages.dev/features/ai/ai-errors/
2. Read through and confirm everything is covered / readable.

Quality assurance

• Security - I have thought about any security implications this code might add.
• Performance - I have checked that this code doesn't impact performance (greatly).
• Caching - I have analyzed the caching methods that this code touches and have added instructions to deal with those.
• Tested - I have tested this code to the best of my abilities.
• Automated tests - I have added unit tests to verify the code works as intended.
• Testability - I have added unique ids to elements, so they can be located in automated testing.
• I have altered a filename.
  • I have adjusted the ID property accordingly and updated all internal links.
  • I have added the redirect to the _redirects file in the root of the project.

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[mhkuu]

Nice to see some documentation for this! 🎉 Here are some initial suggestions.

I would love to see more information on other possible errors:

• site unreachable
• inactive license
• generic errors
• ... basically every error listed in assets/js/src/ai-generator/components/errors

[marinakoleva]

• Swapped all mentions of "alert" or "alert modal" to "error message" (slack thread)
• We could add slightly bigger images so that the error text is easier to read. I changed the first image as an example.
• We could also add the text of the error to make it possible to copy it when needed, or to help with reading it. Again, an example was added to the first error.
• Because the text of some error messages seems a bit unclear/ unhelpful, another task was created to improve them: Update copy and links of error messages
• Crucially, I wonder whether this page is necessary to exist, given that we already have the page Common AI feature problems and errors on yoast.com. What we could do instead, is revamp the yoast.com page, and add the information and images on this page there.