Remove ScrollSpy safeIds workaround and related docs

[chalin]

• Followup to Add ScrollSpy patch workflow and runtime patch #2382
• Fixes TOC scrolling results in runtime error when heading ID starts with a digit #2329. I'm going to consider the issue fixed.

  NOTE: If/once ScrollSpy gets fixed, we can remove the patching code added via Add ScrollSpy patch workflow and runtime patch #2382

• Carefully reverts the relevant parts of Implement a workaround to ScrollSpy's heading-ID problem #2370 while preserving fix: ensure ScrollSpy can always find the TOC #2373.
• Eliminates the temporary safeIds workaround for ScrollSpy issues with heading IDs starting with digits. Cleanes up related documentation, configuration, and template logic now that the workaround is no longer needed.