Fix grammar and reorganize need_for_speed.md content

- Fix incomplete sentence: add missing word "compiler" (line 229)
- Fix header level inconsistency: change Multi-GPU Servers to #####
- Reorganize Overview section with clearer structure
- Simplify Python's Scientific Ecosystem section
- Restructure "Pure Python is slow" section for better flow
- Add concrete vectorization speed comparison example
- Improve parallelization section organization
- Clarify GPU/TPU accelerator discussion
- Remove redundant content and improve transitions throughout

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: jstac