[docs] Add a "configuration errors" troubleshooting page

Description Megatron Core validates many interactions between tensor, pipeline, context, expert, and sequence parallelism. The resulting errors are useful individually, but users lack a central explanation of how to diagnose them. Add a short troubleshooting page covering 5-8 common validation failures already present in current code. For each:

• Show a minimal invalid configuration.
• Include the relevant error text or a stable excerpt.
• Explain the violated invariant.
• Show one corrected configuration.

Candidate topics include divisibility requirements, parallel-size relationships, virtual pipeline configuration, and incompatible precision options.

Do not document runtime CUDA/NCCL failures or performance tuning in this issue.

Acceptance criteria

• Every example is verified by a CPU-only config-construction test or existing unit test.
• Examples use current configuration field names.
• Error excerpts are specific enough to search for but do not depend on line numbers.
• The page is linked from the user guide troubleshooting section.