Feature/elevation

[kate-gladeye]

• Added elevation levels and props to components which should have them
• Set all modals to use overlay
• Added documentation

TODOs:

Design:

• Add color tokens for all background, border and shadow for each elevation
• Add color tokens for bottom and top elevation as well - these are used in a couple of places where a component has nested elevations, like the table header in table. Might need renaming?
• Once the correct colors are added, check that they make sense across all use-cases. Maybe some components need specific adjustments?

Dev:

• Added temporary elevation colors to package/ui/src/theme-base.css with a TODO comment - they need to be added as color tokens instead!
• dropdown-menu.module.css and select.module.css require the use of --mix-color but this requires the CSS var which we haven't set yet - added TODO comments to these as they will need to be adjusted after the tokens are added!