[clang-tools-extra][docs] Add documentation for clang-reorder-fields (#178446)
Add comprehensive documentation for the clang-reorder-fields tool,
addressing #35520. The tool has existed in the repository but was
previously undocumented.
The documentation includes:
- Basic usage examples for C and C++ structs/classes
- Constructor initializer list reordering
- Designated initializer support (C++20)
- Detailed limitations and caveats
- Command line option reference
- Common use cases (memory layout optimization, etc.)
Fixes #35520
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Co-authored-by: EugeneZelenko <eugene.zelenko@gmail.com>