docs(turbopack): Move mdbook "tasks" docs over to `turbo_tasks::function` rustdoc #91132

Performance Regression: -3.15%

❌ 1 regressed benchmark
✅ 16 untouched benchmarks
⏩ 3 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

	Mode	Benchmark	`BASE`	`HEAD`	Efficiency
❌	Simulation	`packages-bundle.js[full]`	1.2 s	1.2 s	-3.15%

_{Comparing bgw/turbo-tasks-function-docs (f8c4901) with canary (8850163)}

3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩