doc: add dataclass reflection guide and update related docs (#494)

## Summary

Add a comprehensive guide for the dataclass-style reflection system
(`ObjectDef`, field traits, auto-init, deep copy, repr, hashing,
comparison) and update the Sphinx documentation configuration and
cross-references.

## Changes

### New documentation

- **`docs/guides/dataclass_reflection.rst`** — A 467-line guide
covering:
- Quick start: defining a C++ object with `ObjectDef`, using it from
Python with full dataclass semantics.
- Auto-generated constructors: how `__ffi_init__` is synthesized from
reflected fields, including positional, optional, and keyword-only
parameter ordering.
- Field traits reference: `refl::default_()`, `refl::default_factory()`,
`refl::kw_only()`, `refl::init()`, `refl::repr()`, `refl::hash()`,
`refl::compare()`, `refl::Metadata`.
- Dataclass operations: `DeepCopy`, `ReprPrint`, `RecursiveHash`,
`RecursiveEq` with behavioral details (iterative DFS, cycle handling,
DAG caching).
- Custom hooks via `TypeAttrDef` for hash, equality, comparison, and
repr overrides.
  - Python `c_class` decorator and `@register_object` delegation.
- Inheritance semantics for field composition across parent/child object
hierarchies.

### Sphinx configuration updates (`docs/conf.py`)

- Exclude `tvm::ffi::reflection::default_` and
`tvm::ffi::reflection::default_factory` from Doxygen symbol output
(these are user-facing helpers that cause Doxygen parse noise).
- Fix `_link_inherited_members` to rewrite `.CObject` references as
`.Object` in cross-reference links, correcting broken autodoc
inherited-member links.
- Add `_move`, `__move_handle_from__`, and
`__init_handle_by_constructor__` to `_autodoc_always_show`, ensuring
these low-level Object lifecycle methods appear in auto-generated API
docs.

### Cross-reference fixes in existing docs

- **`docs/concepts/object_and_class.rst`**: Update
`refl::DefaultValue(0)` to `refl::default_(0)` to match the current
preferred API name.
- **`docs/guides/export_func_cls.rst`**: Replace broken
`:cpp:class:\`tvm::ffi::reflection::init\`` Sphinx cross-reference with
inline code literal `` ``init<Args...>()`` `` (the template alias does
not resolve as a Doxygen class).

### TOC integration

- **`docs/index.rst`**: Insert `guides/dataclass_reflection.rst` into
the Guides toctree between `export_func_cls` and `kernel_library_guide`.

## Behavioral changes

None. This is a documentation-only change with no code modifications.

## Test evidence

Documentation-only PR; no runtime behavior is affected. The guide's code
examples are consistent with the existing C++ reflection API
(`ObjectDef`, field traits) and Python `c_class`/`register_object`
implementations validated by the existing test suite
(`tests/python/test_dataclass.py`, `tests/cpp/test_reflection.cc`).

### Untested corner cases

- Sphinx build with the new RST file has not been verified in CI yet
(will be validated by the doc-build CI job).
- The `.CObject` -> `.Object` rewrite in `_link_inherited_members` is a
string replacement; edge cases where a class legitimately contains
"CObject" in its qualified name would be incorrectly rewritten (none
exist in the current codebase).

Author: junrushao

docs/concepts/object_and_class.rst

@@ -393,7 +393,7 @@ to register reflection metadata for object type ``T``:
    refl::ObjectDef<MyObjectObj>()
        .def_rw("value", &MyObjectObj::value,
                "The numeric value",                    // docstring
-               refl::DefaultValue(0),                  // default value
+               refl::default_(0),                       // default value
                refl::Metadata{{"min", 0}, {"max", 100}})  // custom metadata
        .def("add_to_value", &MyObjectObj::AddToValue,
             "Add a value to the object's value field");

docs/conf.py

@@ -92,7 +92,8 @@
                           TVM_FFI_EXTRA_CXX_API= TVM_FFI_WEAK= TVM_FFI_DOXYGEN_MODE \
                           __cplusplus=201703
 EXCLUDE_SYMBOLS        += *details*  *TypeTraits* std \
-                         *use_default_type_traits_v* *is_optional_type_v* *operator*
+                         *use_default_type_traits_v* *is_optional_type_v* *operator* \
+                         tvm::ffi::reflection::default_ tvm::ffi::reflection::default_factory
 EXCLUDE_PATTERNS       += */function_details.h */container_details.h
 ENABLE_PREPROCESSING   = YES
 MACRO_EXPANSION        = YES
@@ -292,6 +293,8 @@ def _link_inherited_members(app, what, name, obj, options, lines) -> None:  # no
     if base in _py_native_classes or getattr(base, "__module__", "") == "builtins":
         return
     owner_fq = f"{base.__module__}.{base.__qualname__}".replace("tvm_ffi.core.", "tvm_ffi.")
+    if owner_fq.endswith(".CObject"):
+        owner_fq = owner_fq.removesuffix(".CObject") + ".Object"
     role = "attr" if what in {"attribute", "property"} else "meth"
     lines.clear()
     lines.append(
@@ -338,6 +341,9 @@ def _import_cls(cls_name: str) -> type | None:
     "__ffi_init__",
     "__from_extern_c__",
     "__from_mlir_packed_safe_call__",
+    "_move",
+    "__move_handle_from__",
+    "__init_handle_by_constructor__",
 }
 # If a member method comes from one of these native types, hide it in the docs
 _py_native_classes: tuple[type, ...] = (