[Docs] Fix EuiMarkdownEditor plugin example polluting other examples

[cee-chen]

Summary

The previous example code was polluting the default link validator plugin, leading to https://eui.elastic.co/v94.2.0/#/editors-syntax/markdown-format#link-validation-for-security incorrectly not parsing http links. This PR fixes the example to use the new exclude configs we added in #7676, and is overall a significantly cleaner approach in any case.

QA

• Confirm https://eui.elastic.co/pr_7929/#/editors-syntax/markdown-plugins#link-validation-security still does not have working http links
• Confirm https://eui.elastic.co/pr_7929/#/editors-syntax/markdown-format#link-validation-for-security has all working links, including http ones

General checklist

N/A, docs only change

[elasticmachine]

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