docs: update TensorRT RTX EP documentation for standalone EP ABI

[ishwar-raut1]

Summary

Updates the TensorRT RTX Execution Provider documentation to reflect the transition
to the standalone EP ABI plugin and updated default behavior.

Changes

• Deprecation notice: Added a prominent deprecation notice for the built-in TensorRT RTX EP,
  directing users to the standalone TensorRT RTX EP ABI plugin.
• Standalone EP ABI usage instructions: Added C++ code examples demonstrating the V2
  device-based EP API (RegisterExecutionProviderLibrary, GetEpDevices,
  SessionOptionsAppendExecutionProvider_V2) for registering and using the standalone plugin.
• Build from source: Split into "Standalone EP ABI Plugin (Recommended)" and
  "Built-in EP (Deprecated)" sections with appropriate links.
• CUDA Graph default updated: Updated documentation to reflect that enable_cuda_graph
  is now enabled by default (true). Code examples and onnxruntime_perf_test commands
  now show how to disable CUDA Graph rather than enable it.
• onnxruntime_perf_test usage: Added EP ABI plugin command-line examples using
  --plugin_eps, --plugin_ep_libs, and --plugin_ep_options flags alongside the
  deprecated built-in EP commands for CUDA Graph, EP context generation, and general
  performance testing sections.
• EP options note: Clarified that all execution provider options apply to both
  the built-in EP and the standalone EP ABI plugin.

Motivation

The built-in TensorRT RTX EP is being deprecated in favor of the standalone open-source
EP ABI plugin. This documentation update ensures users have clear migration guidance,
up-to-date code examples, and accurate default values.

[ishwar-raut1]

@chilo-ms to review

[ishwar-raut1]

@cloudhan @snnn @faxu @devang-ml