Draft PR for Issue #486: Drop Down Component

[zvidmarb]

Fixes #486

What changes did you make?

• Removed dropdown-dedicated color variables for now
• Began dividing component into its design atoms and their separate doc pages (i.e. removed dropdowns.md; added menu-items.md and dropdown-base.md)
• menu-item.md was added with basic Mkdocs structuring and initial layout of the Menu Item class to be reused for its states and variants.
• dropdown-base.md was added, copying the structuring from menu-item.md, but no actual implementation.
• added sidebar navigation items for dropdown and one of its atoms ("Atoms: Menu Item")
• replaced all styling in _dropdowns.scss; partially updated with styling for a single menu-item variant (with new class names), not complete.

Why did you make the changes (we will use this info to test)?

• Changed Mkdocs structure and naming conventions to be more consistent with other pages (e.g. Buttons Buttons #10 or Accordion Accordion Component PR #14)
• Previous contributions didn't reflect most up-to-date design microsite pages Figma, Dropdowns Microsite Page
• Progress stopped on Dropdown base while input fields implementation is finished (PR for Issue #751: Input Fields component #16); input fields are visually similar to this atom
• Design bundles break this component down into many different atoms, determine best approach of implementation for these microsites from a dev perspective

Screenshots of Proposed Changes To The Website (if any, please do not include screenshots of code changes)

Visuals before changes are applied

Visuals after changes are applied