[EuiPortal docs] Add custom flyout example

[elizabetdev]

Summary

Closes #6256.

We normally recommend in our slack to override a EuiFlyout or to create one from scratch using a EuiPortal. We always point to what should be the right a11y features but we don't have an example in our docs.

So this PR, hopefully, adds an example of what a custom flyout can be with a recommend a11y.

The example can be found here: https://eui.elastic.co/pr_6247/#/utilities/portal#a-custom-flyout

Checklist

• [x] Checked in both light and dark modes
• [ ] Checked in mobile
• [x] Checked in Chrome, Safari, Edge, and Firefox
• [ ] Props have proper autodocs and playground toggles
• [ ] Added documentation
• [ ] Checked Code Sandbox works for any docs examples
• [ ] Added or updated jest and cypress tests
• [ ] Checked for breaking changes and labeled appropriately
• [ ] Checked for accessibility including keyboard-only and screenreader modes
• [ ] Updated the Figma library counterpart
• [ ] A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6247/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6247/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6247/

[elizabetdev]

jenkins test this

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6247/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6247/