[VitisAI][Documentation] Update VitisAI EP doc (#24739)
### Description
- Remove references to development environment variables
- Update Provider options:
- `cacheDir` -> `cache_dir`
- `cacheKey` -> `cache_key`
- added `log_level`
- removed development option `config_file`
- Update C++ example code
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Co-authored-by: liumingyue <mingyue@xilinx.com>
