[Layout foundations] Update `AlphaStack` component docs and guidance

[laurkim]

Update AlphaStack docs

• Description
• Examples: descriptions and visuals
• Add related components section

Update
Description	Use to display elements vertically with items placed directly below the preceding item.
Default example description	By default, stack is left aligned and stacked items are spaced with 16px in between.
Spacing example description	Spacing between stack child can be adjusted using the Space token.
Full width example description	Use a boolean to make Stack full width.
Alignment example description	Use to vertically align Stack.
Related components	To display elements horizontally, use Inline.

Before and after

Link to Figma