[EUI+] Docasaurus theme styles affect EUI examples

[weronikaolejniczak]

The purpose of this task is to make the Docasaurus theme styles not affect the EUI examples.

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Docusaurus theme styles are leaking into our EUI examples and making them not look exactly as they should in production, which is going to be misleading / confusing for consumers.

Example elements:

• <hr>
• <h[1-6]>
• <a> hover / active colors