Restyle the docs to the liquid.ai look - #167

Refines the docs to match liquid.ai and makes mobile navigation and feedback clicks clearer. Page URLs, navigation destinations, and notebook links are preserved.

Changes

• Article headers: 32px serif titles in black for light mode and white for dark mode, tighter spacing below the title and header, and smaller Copy page controls.
• Navigation: collapsible sidebar sections, narrower right rails, and an always-visible native close button inside the mobile drawer’s sticky header.
• Shared components: 2px corners, thinner card borders, flatter code blocks and callouts, neutral table headers and descriptive badges, and revised hover/focus styles. Step numbers and scrollbar handles stay round; availability ticks stay green.
• Model pages: metadata above serif card titles, two-column category grids, format buttons that wrap on phones, and two previously missing capability icons fixed.
• API reference: neutral method pills and Try it controls, violet path parameters, and narrower desktop examples to give the article more room.
• Colab: all six original branded badges retained at 24px high, with their proportions and notebook links preserved.
• Feedback: a two-second tick acknowledges Yes/No clicks while preserving Mintlify’s native voting behavior and labels.
• Maintenance: shared styling guidance moves into DESIGN.md; CSS and its development copy remain synchronized.

Validation

• Docs checks, the link-snapshot check (222 active URLs), and Mintlify deployment pass.
• JavaScript syntax, CSS synchronization, and diff whitespace checks pass.
• Desktop and phone views checked in light/dark mode, including menus, cards, tables, API controls, all Colab badges, and long 32px titles at 320px.
• Mobile close controls and feedback interactions checked in Chromium and WebKit. Feedback tests block outgoing submissions; selection, keyboard activation, timeout, and navigation reset are verified.