docs: Add warning about dangling references with lazy reducers (fixes #2871)

[ssam18]

This PR addresses GitHub issue #2871 where using auto with xtensor reducer functions (like amax, sum with keep_dims) causes crashes or incorrect results in optimized builds, particularly on Android ARM64.

Root Cause: Lazy expressions hold references to local variables. When intermediate results are stored with auto, these references become dangling when the function returns, leading to undefined behavior.

Changes:

• docs/source/pitfall.rst: Added detailed warning section explaining the issue and providing three solutions:
  1. Use explicit container types (xt::xtensor<T, N>)
  2. Use xt::eval() to force immediate evaluation
  3. Use evaluation_strategy::immediate with reducers
• test/test_xmath.cpp: Added test demonstrating correct patterns

Testing:

• All 83 xmath tests pass
• All 64 xreducer tests pass

Fixes #2871