Remove RTOS API content from site

[espmaniac]

What changed

• remove the RTOS API navigation item and hero CTA
• remove the synchronization, timing, and API reference section
• simplify the hero copy and page metadata so they no longer advertise the API
• remove the now-unused API-section styles
• keep a single source-code CTA and correct the reduced mobile navigation

Why

The site should focus on the live WASM runtime and task dashboard instead of duplicating RTOS API documentation.

Validation

• node --check main.js
• git diff --check
• confirmed that no API references or API-section selectors remain in the rendered page
• exact public commit preview reached Runtime: OK
• verified navigation, hero CTA, styling, and absence of horizontal overflow
• GitHub Actions Build browser runtime run #69 completed successfully