docs: add public API documentation

[voltjia]

Summary

• Add public user documentation for README.md, docs/README.md, docs/build.md, docs/api/runtime.md, docs/api/core-types.md, docs/backends.md, and docs/compatibility.md.
• Add runtime usage examples under examples/, including a minimal installed-consumer CMake project.
• Add the INFINI_RT_BUILD_DOCS CMake option and infinirt_docs Doxygen target.
• Document how to preview the generated Doxygen HTML structure locally.

Motivation

InfiniRT now has a public runtime surface, but users need a documented entry point, build instructions, example consumers, and a generated API reference path. This PR establishes that documentation baseline while keeping GitHub Pages integration out of scope for a follow-up PR.

Closes N/A.

Type of Change

• feat — new feature / new operator / new platform
• fix — bug fix
• perf — performance improvement (no behavioral change)
• refactor — code restructuring without behavior change
• test — adding or fixing tests only
• docs — documentation only
• build / ci — build system or CI configuration
• chore — tooling, formatting, or other non-code changes
• Breaking change (requires a ! in the Conventional Commits prefix or a BREAKING CHANGE: footer)

Platforms Affected

• CPU (WITH_CPU)
• NVIDIA (WITH_NVIDIA)
• Iluvatar (WITH_ILUVATAR)
• MetaX (WITH_METAX)
• Cambricon (WITH_CAMBRICON)
• Moore (WITH_MOORE)
• Ascend (WITH_ASCEND)
• PyTorch C++ bindings (WITH_TORCH)
• Build system / CMake / CI
• Python bindings / user-facing API

Smoke Test Result

Markdown relative link check:
Markdown relative links OK

ssh nvidia, clang-format 14:
clang-format --dry-run --Werror \
  examples/basic_runtime.cc \
  examples/select_backend.cc \
  examples/tensor_view.cc \
  examples/consumer_cmake/main.cc
Result: passed

ssh nvidia, Doxygen reference generation:
docker run --rm -v <source>:/workspace -w /workspace \
  accelerator-dev/nvidia:doxygen \
  cmake -S . -B build -DWITH_CPU=ON -DINFINI_RT_BUILD_DOCS=ON
Result: configuring done, Doxygen 1.9.8 found

docker run --rm -v <source>:/workspace -w /workspace \
  accelerator-dev/nvidia:doxygen \
  cmake --build build --target infinirt_docs
Result: Built target infinirt_docs

Generated HTML check:
build/docs/reference/html/index.html exists
index.html contains: Runtime API for Accelerators

Test Results on Supported Platforms

Platform	Affected	Build / Smoke Result	Full Result / Notes
NVIDIA	No	N/A - docs only; Doxygen validation ran on ssh nvidia	N/A - not affected
Iluvatar	No	N/A - not affected	N/A - not affected
MetaX	No	N/A - not affected	N/A - not affected
Cambricon	No	N/A - not affected	N/A - not affected
Moore	No	N/A - not affected	N/A - not affected
Ascend	No	N/A - not affected	N/A - not affected

Full `pytest` output (optional)

N/A - no pytest suite was run because this PR only adds documentation, examples, and the Doxygen documentation target.

Benchmark / Performance Impact

N/A - this PR is not performance-sensitive.

Notes for Reviewers

• GitHub Pages integration is intentionally left for a follow-up PR.
• The generated API reference is scoped to public entry headers, core public headers, and generated dispatching runtime declarations.
• examples/.clang-format inherits the repository style and only classifies <infini/...> includes as third-party headers for example code formatting.