Address PR review comments for build time optimization doc

Changes:
- Move to include/ck/ (developer documentation, not customer-facing)
- Add tracking issue link at top
- Fix section structure (sequential numbering 1-5)
- Remove mismatched transform_tensor_descriptor example
- Clarify O(N) constexpr loop vs template recursion distinction
- Remove "Case Studies" section (redundant with tracking issue)
- Simplify examples for clarity

Author: tenpercent