[docs] feat: document min GPU count for combined dense + MoE parallelism

[cuichenx]

Summary

Updates skills/perf-techniques/parallelism-strategies/SKILL.md with a new Minimum GPU Count section.

The dense path uses a TP*CP-mesh and the MoE path uses an EP*ETP-mesh; within each PP stage these two meshes share the same GPUs rather than multiplying. The correct minimum is therefore:

```
min_gpus = PP * max(TP * CP, EP * ETP)
```

not the common (and incorrect) PP * TP * CP * EP * ETP that shows up in READMEs and slurm sizing tables.

Changes

• New Minimum GPU Count section with the formula, an explicit "wrong" callout, and a worked example table covering common dense + EP combos.
• Extends the existing implicit DP formula to include expert_data_parallel_size = world_size / (PP * EP * ETP) for the MoE path.
• Adds a "Scaling above the minimum" subsection showing how adding GPUs scales DP / EDP.
• Cites Pangu Ultra MoE (arXiv:2504.14960) where the decoupling of attention and MoE meshes is described.
• Adds an entry to the existing Pitfalls list pointing readers at the new section.

Test plan

• Docs-only change; no runtime impact.
• Pre-commit passes (markdown is not linted; whitespace / EOF hooks skip).