fix(PPDSC-2634): select panel zindex

[mutebg]

PPDSC-2634

What

1. Background - the select panel goes over other elements with fixed position and z-index -> https://codesandbox.io/s/busy-stonebraker-rdxer2?file=/src/App.tsx
2. What did you do - the select panel is rendered inside a layer organizer, which means it has its own stacking context and is on top of everything. However, there are some cases you want to have a fixed header or other elements on top of that. In order to solve this I added zIndex as an override to the panel. By default uses the keyword "layer" which keeps the current behaviour ( rendering in a layer organizer ) but users can override that by passing a z-index value which applies to the panel.
3. What does the reviewers should expect

I have done:

• Written unit tests against changes
• Written functional tests against the component and/or NewsKit site
• Updated relevant documentation

I have tested manually:

• The feature's functionality is working as expected on Chrome, Firefox, Safari and Edge
• The screen reader reads and flows through the elements as expected.
• There are no new errors in the browser console coming from this PR.
• When visual test is not added, it renders correctly on different browsers and mobile viewports (Safari, Firefox, small mobile viewport, tablet)
• The Playground feature is working as expected

Before:

After:

Who should review this PR:

How to test:

[newskit-engineering]

You can preview these changes on:
Documentation Site: http://ncu-newskit-docs.s3-website-eu-west-1.amazonaws.com/ppdsc-2634-select-panel-zindex
Storybook: http://ncu-newskit-docs.s3-website-eu-west-1.amazonaws.com/ppdsc-2634-select-panel-zindex/storybook