Improve docs navigation and align the visual theme

[styler]

The docs need room for SE.Live and more product guides. This adds a compact product selector above each product's sidebar, keeps Guides and reference groups collapsible, and keeps the selector visible while scrolling. Existing documentation URLs remain intact.

Adds SE.Live starter pages and a Developers landing page, with matching product links on the homepage. Aligns both themes with frontend/packages/ui, uses the official SE logo, and refines header controls, typography, card layouts, callouts, chat contrast, table scrolling, and mobile navigation.

Validation:

• pnpm build: 203 pages built; internal links pass.
• git diff --check: passes.
• Chrome visual review in light and dark themes, including search, dropdowns, images, code, tables, and pagination.
• 63 layout checks across 21 routes at 320px, 768px, and 1280px: no page overflow, missing headings, or broken loaded images. Additional checks at 390px and 820px covered mobile controls and the sidebar breakpoint.

Theme tokens and SVGs are copied into this repo; builds do not require the frontend checkout. SE.Live starter pages link to the Help Center for the fuller product guides.

[linear-code]

CORE-1471