docs: add guide for extending NVQIR simulator

[leodiasdc]

Fixes #3103. See #3099 for the original report of the two specific bugs
(stale MySimulator.cpp skeleton, missing GPURequirements argument to
nvqir_add_backend).

What was wrong

The using/extending/nvqir_simulator.rst page had drifted significantly
from the current CircuitSimulatorBase interface and nvqir_add_backend
CMake helper:

• nvqir_add_backend(...) was missing the GPURequirements argument.
• The example MySimulator.cpp referenced methods that no longer exist
  (resetQubitStateImpl), used a return type that no longer exists
  (cudaq::SampleResult), and called NVQIR_REGISTER_SIMULATOR with one
  argument instead of two.
• The "Required Method Overrides" table was missing createStateFromData
  and clone (both required), and listed addQubitsToState as required
  even though it now has a default implementation.
• The target-config section described a hand-authored NAME.config under
  /platforms; NVQIR now generates a .yml file under /targets
  automatically.

What this PR does

• Corrects all of the above against the current
  runtime/nvqir/CircuitSimulator.h and cmake/modules/NVQIRConfig.cmake.in.
• Replaces the inline, stale code block with a real, working, minimal dense
  state-vector simulator (docs/sphinx/snippets/cpp/using/extending/nvqir_simulator/),
  pulled into the doc via literalinclude instead of hand-copied prose.
• Adds a "Toolchain requirements" section covering two things I hit while
  testing that weren't documented at all: the C++20 requirement, and the
  fact that CUDA-Q's binaries are built with Clang+libc++, so a default
  system g++/libstdc++ build configures and compiles fine but fails at
  load time with a symbol lookup error.
• Adds a "Testing your simulator" section with a Bell-state smoke test.

Testing

Built and ran against a real CUDA-Q 0.15.0 install:

```
$ cmake .. -G Ninja -DNVQIR_DIR=... -DCMAKE_CXX_COMPILER=.../clang++ -DCMAKE_CXX_FLAGS=-stdlib=libc++
$ ninja install
-- Installing: .../lib/libnvqir-MySimulator.so
-- Installing: .../targets/MySimulator.yml

$ nvq++ bell.cpp --target MySimulator -o bell && ./bell
{ 00:471 11:529 }
```

(Bell state: h + ctrl-x + mz on 2 qubits, confirming applyGate,
measureQubit, and sample are all correct, not just that it compiles.)

Open question for maintainers

I added a small compile-only target in docs/CMakeLists.txt that builds
MySimulator.cpp in-tree against the nvqir target, so this doc can't
silently drift again. I wasn't able to validate this against a full
in-tree build in my environment — happy to drop it if it causes ordering
issues, or if there's a preferred place for this kind of doc-example
compile check.

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