docs(qec): standalone decoding server reference page

[vedika-saravanan]

Description

Adds docs/sphinx/examples_rst/qec/decoding_server.rst, a new
reference page for the standalone decoding_server binary introduced
in the realtime decoding server work.

• Transports: table covering udp (CI/dev), cpu_roce (SoftRoCE /
  HSB FPGA HOST_CALL), and gpu_roce (DOCA + Hololink device-graph
  scheduler), with when-to-use guidance for each.
• YAML configuration: full field reference including transport and
  cuda_device_id; Python binding note for cuda_device_id; note on
  orchestration script auto-injection.
• GPU affinity reconciliation: four-case table for
  cuda_device_id vs HOLOLINK_GPU_ID resolution at startup.
• Building: prerequisites, CMake flags, private graph-dispatch
  symbols (decoder_rpc_dispatch.cu / cudaq-qec-realtime-decoder-rpc-dispatch
  whole-archive link) with public-build fallback behavior explained.
• Running: CLI reference, readiness signals, and launch examples for
  all three transports.
• GB200 / aarch64 page-size alignment: explains the 64 KiB host-page
  constraint, the 512 x 64 = 32768 failure case, and the
  --gpu-roce-num-pages auto fix.
• Orchestration script (hsb_fpga_decoding_server_test.sh): full
  option reference (modes, actions, decoder options, data generation,
  run options, network options, build options) with quick-start examples.
• Troubleshooting: covers DOCA allocation failures, GPU ID mismatch,
  missing proprietary symbols, ILA deterministic verification fix
  (expected sample count vs. stabilization heuristic), and common
  network issues.

Linked from the QEC examples toctree (examples.rst) and the
realtime decoding section of introduction.rst.

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Runtime / performance impact

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Before requesting review

• I reviewed my own full diff in GitHub or my editor.
• PR is in Draft if it is not yet ready for review.
• Temporary / debugging changes have been removed.
• Local test logs reviewed; no unexplained warnings or errors.
• CI logs reviewed; no unexplained warnings or errors.
• Full CI has been run.

Scope and size

• PR is under ~1000 lines, or an exception is justified in the description.
• Refactoring-only changes are isolated in their own PR(s).
• No existing tests were disabled or modified just to make this PR pass
  (if so, an issue has been raised).

Tests

• New functionality has new tests.
• Tests fail if the new functionality is broken (including crashes), not
  just when it is missing.
• Negative tests added where exceptions are expected.
• Truth data added where simple EXPECT_* / assert checks are
  insufficient for algorithmic correctness.
• CI runtime impact considered; team notified if significant.

Documentation

• Public-facing APIs have Doxygen docs.
• User-visible behavior changes have public docs, or a follow-up is
  tracked.

Code style

• Naming follows the existing convention (snake_case vs camelCase) for
  the area being modified.

Dependencies

• No new third-party dependencies, or the team has been notified and
  OSRB tickets filed.