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!

[Shrinks99]

This is a big one! Should be ready to go, component docs still need work, but the elevation properties all seem to be in order! Have also clarified some usage guidelines in the elevation docs page.

Will do a final overview and merge tomorrow.

[tmccoy14]

This looks good to me - only thing I saw were the errors about declaring variables but never using them