docs(EuiEmptyPrompt): Fix missing icons in EUI+ docs

[tkajtoch]

Summary

Resolves #8312

This fixes icon rendering in EUI+ EuiEmptyPrompt docs. Utilizing both iconType and icon props felt easiest to achieve what we want.

QA

Please note that EuiEmptyPrompt guidelines use renderToJSXString, which is incompatible with Docusaurus and our Demo component when used in prod builds. Additionally, EuiPageTemplate's sidebar gets rendered incorrectly as reported here. These pages currently work only when running locally, which is unrelated to this PR.

1. Checkout this PR - gh pr checkout <id> && yarn
2. Build website dependencies - yarn workspace @elastic/eui-website run build:workspaces
3. Run Docusaurus - yarn workspace @elastic/eui-website run start
4. Ensure the EuiEmptyPrompt's Types of empty states example works comparably to the current docs site

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8488/
• Storybook: https://eui.elastic.co/pr_8488/storybook
• New docs: https://eui.elastic.co/pr_8488/new-docs

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 4297756

[weronikaolejniczak]

@tkajtoch there's a reference error:

[tkajtoch]

@weronikaolejniczak I know, I mentioned it in the description. main has the same error and it's related to convertToJsxString usage when code is minified for production. It works locally and will need to be fixed globally.

[weronikaolejniczak]

@tkajtoch ah, I'm sorry, I read the description but didn't connect those two. Let me checkout the branch and test locally then 😄

[weronikaolejniczak]

LGTM! 🎉 Thanks for figuring this out, Tomasz!

Screen.Recording.2025-03-25.at.12.07.33.mov