docs: add contribution guidelines

[voltjia]

Summary

• Add a repository-local CONTRIBUTING.md adapted for InfiniRT development.
• Add .github/PULL_REQUEST_TEMPLATE.md with InfiniRT-specific validation and platform sections.
• Link the contributor guide from README.md and docs/README.md.

Motivation

InfiniRT development has been following the InfiniOps contribution and PR conventions, but those conventions were not directly visible or usable from this repository. This change brings the shared workflow into InfiniRT while replacing InfiniOps-specific Python, pytest, and operator guidance with InfiniRT CMake, ctest, Doxygen, and backend guidance.

Closes #

Type of Change

• feat - new feature / new backend capability / new public API
• fix - bug fix
• perf - performance improvement without behavior 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)
• Hygon (WITH_HYGON)
• MetaX (WITH_METAX)
• Moore (WITH_MOORE)
• Cambricon (WITH_CAMBRICON)
• Ascend (WITH_ASCEND)
• Build system / CMake / generated headers
• Public headers / installed consumer API
• Documentation only

Smoke Build and Test Result

git diff HEAD~1 HEAD --check
# passed

rg -n "[^\\x00-\\x7F]" CONTRIBUTING.md .github/PULL_REQUEST_TEMPLATE.md README.md docs/README.md
# no matches

Test Results on Supported Platforms

Platform	Affected	Build / Smoke Result	Full Result / Notes
CPU	No	N/A - documentation only	N/A
NVIDIA	No	N/A - documentation only	N/A
Iluvatar	No	N/A - documentation only	N/A
Hygon	No	N/A - documentation only	N/A
MetaX	No	N/A - documentation only	N/A
Moore	No	N/A - documentation only	N/A
Cambricon	No	N/A - documentation only	N/A
Ascend	No	N/A - documentation only	N/A

Full `ctest` output (optional)

N/A - documentation and PR template only.

Benchmark / Performance Impact

N/A - documentation only.

Notes for Reviewers

The guide intentionally keeps the first step narrow: it adds repository-local contribution and PR workflow documentation, but does not migrate development scripts, clang-tidy configuration, or CI platform configuration. Those can be handled as follow-up changes after this contributor-facing baseline lands.