Add paddingSize prop to EuiCard and display example to docs

[andreadelrio]

Summary

This PR adds the paddingSize prop to EuiCard and the example for the display prop to the docs.

Screenshot of different paddingSizes (s, m and l)

Notes:

• I excluded the option for no padding from paddingSize because I think given the characteristics of the component, it wouldn't make sense to have cards with no padding whatsoever.

• I didn't add an example for paddingSize since I saw that we don't usually do that (except for EuiPanel) but happy to add one if we think it's necessary.

Fixes #3552

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation
• Checked Code Sandbox works for the any docs examples
• Added or updated jest tests
  - [ ] Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3638/

[cchaos]

Thanks for adding the display example. I agree, I don't think we need the padding one and we can eventually rely on the Playground for that.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3638/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3638/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3638/

[cchaos]

Last round of comments.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3638/

[cchaos]

👍 Checked it locally by changing the default value of paddingSize, all the configurations still work (albeit don't all look great), but nothing breaks.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3638/