Docs: refine docs layout to widen content area

[TomShawn]

On a 16" screen the article column rendered at only ~760px because the docs pages inherited the 1200px --global-main-width shared with the marketing pages, leaving wide empty gutters on both sides while the sidebar, TOC and article fought over what was left.

Docs pages now use the full viewport, capped at 1920px so lines stay readable on ultra-wide screens. The TOC moves from a percentage-based col--3 to a fixed 288px column, so every remaining pixel goes to the article instead of inflating the TOC on large screens. The article column grows to fill whatever is left.

This also aligns two stale breakpoints (1440px) in the DocItem and DocRoot layouts with the 1280px breakpoint design-style.scss already uses to hide the TOC. Between 1280px and 1440px the TOC was rendered next to an article still claiming 100% width, which squeezed the TOC down to ~185px.

Measured article width at 1728px viewport: 760px -> 921px.

Current:

Refined:

closes: #ISSUE_Number

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