docs: document graph runtime API

[voltjia]

Summary

• Document graph capture and replay types, functions, lifecycle, and usage in docs/api/runtime.md.
• Add graph capture support status to docs/backends.md.
• Update docs/compatibility.md so documented runtime types are part of the stable user-facing surface.

Motivation

PR #9 added the graph runtime API without corresponding user-facing documentation. This PR closes that documentation gap for the generated public runtime surface.

Closes #

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

git diff --check
# passed
C:\Users\huang\.cache\codex-runtimes\codex-primary-runtime\dependencies\python\python.exe -c "... markdown relative link check ..."
Markdown relative links OK

cmake -S . -B build-docs -DWITH_CPU=ON -DINFINI_RT_BUILD_DOCS=ON was not run in this Windows workspace because cmake is not available on PATH. The previously used remote ssh nvidia validation path was also unavailable here because the nvidia host alias did not resolve.

Test Results on Supported Platforms

Platform	Affected	Build / Smoke Result	Full Result / Notes
NVIDIA	No	N/A - docs only	Not run
Iluvatar	No	N/A - docs only	Not run
MetaX	No	N/A - docs only	Not run
Cambricon	No	N/A - docs only	Not run
Moore	No	N/A - docs only	Not run
Ascend	No	N/A - docs only	Not run

Full `pytest` output (optional)

N/A - documentation-only change.

Benchmark / Performance Impact

N/A - documentation-only change.

Notes for Reviewers

The graph API documentation intentionally describes that generated graph entry points depend on the configured build including a graph-capable backend.