docs/sphinx: create QAPI domain extension stub

A Sphinx domain is a collection of directive and role extensions meant
to facilitate the documentation of a specific language. For instance,
Sphinx ships with "python" and "cpp" domains. This patch introduces a
stub for the "qapi" language domain.

Please see https://www.sphinx-doc.org/en/master/usage/domains/index.html
for more information.

This stub doesn't really do anything yet, we'll get to it brick-by-brick
in the forthcoming commits to keep the series breezy and the git history
informative.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-4-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/sphinx/qapi_domain.py

@@ -0,0 +1,56 @@
+"""
+QAPI domain extension.
+"""
+
+from __future__ import annotations
+
+from typing import (
+    TYPE_CHECKING,
+    AbstractSet,
+    Any,
+    Dict,
+    Tuple,
+)
+
+from sphinx.domains import Domain, ObjType
+from sphinx.util import logging
+
+
+if TYPE_CHECKING:
+    from sphinx.application import Sphinx
+
+logger = logging.getLogger(__name__)
+
+
+class QAPIDomain(Domain):
+    """QAPI language domain."""
+
+    name = "qapi"
+    label = "QAPI"
+
+    object_types: Dict[str, ObjType] = {}
+    directives = {}
+    roles = {}
+    initial_data: Dict[str, Dict[str, Tuple[Any]]] = {}
+    indices = []
+
+    def merge_domaindata(
+        self, docnames: AbstractSet[str], otherdata: Dict[str, Any]
+    ) -> None:
+        pass
+
+    def resolve_any_xref(self, *args: Any, **kwargs: Any) -> Any:
+        # pylint: disable=unused-argument
+        return []
+
+
+def setup(app: Sphinx) -> Dict[str, Any]:
+    app.setup_extension("sphinx.directives")
+    app.add_domain(QAPIDomain)
+
+    return {
+        "version": "1.0",
+        "env_version": 1,
+        "parallel_read_safe": True,
+        "parallel_write_safe": True,
+    }