[Docs] Add TIRx documentation section (#19855)
Adds a standalone TIRx documentation section under `docs/tirx/` — the
kernel-level compiler structure (DSL + tile primitives) that ships
inside Apache TVM as `tvm.tirx`.
**Contents**
- Overview, installation, and the storage-first tensor-layout model
(with an embedded interactive layout explorer).
- "Native basics" walkthrough for CUDA C++/PTX-level authoring (threads,
scopes, buffers, sync, compiling).
- Per-dispatch tile-primitive walkthroughs (`copy`, `copy_async`,
`gemm`/`gemm_async`, `reduction`, `elementwise`, `permute_layout`).
- Architecture (lowering pipeline) and the `tvm.tirx` Python API
reference (autodoc).
- Wiring: `docs/index.rst` toctree,
`docs/reference/api/python/index.rst`, `docs/conf.py`, and
`docs/_static/css/tirx_theme.css`.
**Scope:** docs-only — 46 files, all under `docs/`; no source changes.
The autodoc targets (`tvm.tirx`, `tvm.backend.cuda`, ...) already exist
in `main`.
Bohan Hou
15f62fe020