docs: add documentation skeleton

[voltjia]

Summary

• Adds a top-level documentation index in docs/README.md.
• Adds first-pass pages for getting started, build/test commands, backends, core types, operators, and compatibility boundaries.
• Updates README.md to point users to the structured documentation entry points.

Motivation

InfiniOps only had a README, CONTRIBUTING guide, and the ATen operator guide as documentation entry points. This PR creates the first structured documentation skeleton so follow-up documentation work can land page by page without expanding the README.

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

N/A - documentation-only change. No platform code, build logic, generated wrappers, or runtime behavior changed.

$ git diff HEAD~1..HEAD --check
<no output>

$ rg -n "[^\\x00-\\x7F]" README.md docs
<no output>

Test Results on Supported Platforms

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

Full `pytest` output (optional)

N/A - documentation-only change.

Benchmark / Performance Impact

N/A - documentation-only change.

Notes for Reviewers

This PR intentionally does not add Doxygen generation, GitHub Pages publishing, or consumer CMake examples. Those should land as separate follow-up PRs so each step remains reviewable.