docs: align packing guidance with Phase 2 rules

[spMohanty]

The cost-model reference called packing “not banned and not judged” and sub-32-bit packing gains “in-bounds”, contradicting the Phase 2 fair-accounting rules.

This removes both statements and makes the official Challenge Rules authoritative for eligibility. It distinguishes accounting advantages from ordinary lower precision or quantization, explains that metering every operation does not grant permission, and qualifies related claims that pricing eliminates packing gains. The competition guide links to the corrected guidance. The website cost-model page is generated from the updated Markdown source.

The documentation audit covered Markdown and MDX guidance across the repository. New policy wording uses the publicly documented packed-boolean example. No runtime, dtype pricing, or dependency changes.

Validation: 11 tests passed (tests/test_cost_model_page.py and tests/test_cost_model_coverage.py); generated the website page and confirmed the stale permission statements are absent; checked the guide's target route and packing anchor; git diff --check passed.

Based on freshly fetched origin/main at 33cf0c05d3. Companion starter-kit clarification: AIcrowd/whest-starterkit#44.