fix(website): update the button icon to align with the guideline

[weronikaolejniczak]

Summary

Resolve #8855

The guideline example on the Button page didn't align with what the guideline said.

Why are we making this change?

We are making this change because the current example goes against the guideline:

Stick to the default pattern: a filled, primary button paired with an empty, but same-colored button.

EuiButton uses color primary and EuiButtonIcon uses color danger.

We settled with the designers on changing the icon to gear and color to primary.

Screenshots

Before	After

Impact to users

🟢 Affects our documentation website. It's not a breaking change.

QA

• Size guideline example is aligned with the guideline text

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 38e2ee8
• Documentation website
• Storybook

cc @weronikaolejniczak

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 38e2ee8

cc @weronikaolejniczak

[mgadewoll]

🟢 LGTM