Merge pull request mdn#5987 from caugner/improve-404-and-search-layout

chore(ui): migrate 404 and search to page layout

fix(mandala): make 4th circle spin (mdn#5941)

The 4th circle of MDN Plus product page's Mandala SVG didn't
rotate because animateTransform's from and to were the same.

chore(pwa): manage ContentStatus in IndexedDB, but be backward-compat…

…ible (mdn#5980)

Phase 1 of 3.

The ServiceWorker starts to use IndexedDB, but still accepts the
local version from the client as a fallback on the update message,
and still emits messages to inform clients about the status.

For the ServiceWorker, however, the IndexedDB is the truth.

Merge pull request mdn#6010 from caugner/fix-bcd-symbols-in-history

fix(bcd): fix some regressions

fix(bcd): prioritize current support using a ranking system (mdn#5946)

chore(ServiceWorkerSidebar): remove obsolete interfaces (mdn#5965)

chore(macros): deprecate h3_gecko_minversion (mdn#5995)

chore(ServiceWorkerSidebar): add Japanese translation (mdn#5964)

refactor(lru): use delete instead of deprecated del (mdn#5992)

fix(a11y): improve accessibility, readability and usability (mdn#5710)

Accessibility, readability and usability:

- Adjust colors, font-sizes, font-weights and spacing for readability.
- Adjust color and text-decoration of links.
- Make top navigation sticky on desktop.
- Fallback to regular caret when user prefers reduced-motion. 
- Restore skip to search.
- Replace notes symbol in BCD table.

Other changes:

- Improve CSS variable structure.
- Define base font-size and line-height.
- Build contributor-spotlight in dev-build.
- Remove scrollbar customisations.
- Extract common `<Menu> `component.
- Replace `<em>` with `<strong>` in macros.
- Use `<summary>` in JSRef macro.
- Use `<h3>` in APIListAlpha and ListGroups macros.

Co-authored-by: Claas Augner <caugner@mozilla.com>