v6: Docs design updates

[mdo]

Bootstrap

• Nav overflow: added collapseBelow, iconPlacement, and menuPlacement options, plus support for custom icons via [data-bs-overflow-icon] in markup
• Nav pills: now inline-flex with pill border-radius, background color, and padding tokens—more unique from base .nav styling.
• Tables: new .table-stacked classes using container queries for responsive stacked rows; switched striped/active/hover bg-factors to percentages with color-mix() to fix them.
• Form range: added hover state with subtle focus ring, switched focus to :focus-visible, updated thumb border and box-shadow
• Menu: added open/close CSS transitions with @starting-style and per-placement transform-origin
• Dialog: added --dialog-transition-duration and --dialog-transition-timing CSS vars that were missing
• Drawer: removed align-items: center from navbar expand layout, unsure why we needed it.
• Utilities: added grid-cols-1 value, increased available spacer scale from 1-5 to 1-9 for smaller and even/linear .25rem steps

Docs

• New page metadata component and frontmatter: added css_layer, css_media, js, deps, and mdn frontmatter fields across all component, form, and content pages
• Sidebar reorganization and new sidebar/TOC JavaScript
• Moved CSS Variables page from Customize to Getting Started (with alias redirect)
• Deleted unused Interactions utility page
• Various button/chip/menu docs layout fixes (flex-wrap, gap)
• Added dependency metadata to Menu, Popover, Tooltip, Datepicker, Combobox, Navbar
• Docs layout refactoring, new SVG symbols, search and navbar style updates
• Content config schema updates and rehype plugin improvements