Document environment variables

All environment variables must be documented in the central env_var.md file with clear descriptions of their purpose, acceptable values, and effects. Use consistent naming patterns that reflect the subsystem they configure (e.g., MXNET_CUDNN_, MXNET_MKLDNN_) and avoid introducing new variables when existing ones can serve the same purpose.

Example:

// Good - Uses consistent naming and is documented
// In code:
const bool brgemm_disabled = dmlc::GetEnv("MXNET_MKLDNN_DISABLE_BRGEMM_FC", true);

// In docs/static_site/src/pages/api/faq/env_var.md:
// MXNET_MKLDNN_DISABLE_BRGEMM_FC=[true|false] - Disable BRGEMM algorithm in fully connected layers when using MKLDNN backend. Default is true.

// Bad - Introduces new variable without documentation
static bool use_new_dep_engine = dmlc::GetEnv("MXNET_ASYNC_GPU_ENGINE", false);
// Instead, reuse existing variable:
// static bool use_new_dep_engine = (dmlc::GetEnv("MXNET_ENGINE_TYPE", "") == "AsyncGPU");

When adding new configuration options:

1. Check if an existing variable can be extended instead of creating a new one
2. Follow the subsystem naming prefix pattern (MXNET_[SUBSYSTEM]_*)
3. Document the variable in env_var.md with its default value and purpose
4. For enum-like options, document all valid values and their effects