doc: extensions: domain: fix inventory on incremental builds

Generated objects.inv would not contain e.g. "zephyr:board" entry on
incremental builds, making it impossible to use Intersphinx on other
projects when rebuilding the Zephyr docset.

It can be verified by inspecting inventory like this:

```python
from sphinx.util.inventory import InventoryFile
InventoryFile.loads(open("objects.inv", "rb").read(), uri="")
inv.data.keys()
```

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

Author: gmarull

doc/_extensions/zephyr/domain/__init__.py

@@ -1182,6 +1182,13 @@ class ZephyrDomain(Domain):
         "has_code_sample_listing": {},  # docname -> bool
         "board_catalog_docname": None,  # docname of the one page containing the board catalog
         "has_board": {},  # docname -> bool
+        # board catalog data (populated by load_board_catalog_into_domain)
+        "boards": {},
+        "shields": {},
+        "vendors": {},
+        "socs": {},
+        "archs": {},
+        "runners": {},
     }
 
     def clear_doc(self, docname: str) -> None:
@@ -1204,6 +1211,11 @@ def clear_doc(self, docname: str) -> None:
             self.data["board_catalog_docname"] = None
         self.data["has_board"].pop(docname, None)
 
+        # Clear board docnames for boards documented in this docname
+        for board_data in self.data.get("boards", {}).values():
+            if board_data.get("docname") == docname:
+                board_data.pop("docname", None)
+
     def merge_domaindata(self, docnames: list[str], otherdata: dict) -> None:
         self.data["code-samples"].update(otherdata["code-samples"])
         self.data["code-samples-categories"].update(otherdata["code-samples-categories"])
@@ -1419,7 +1431,15 @@ def load_board_catalog_into_domain(app: Sphinx) -> None:
         ),
         hw_features_vendor_filter=app.config.zephyr_hw_features_vendor_filter,
     )
-    app.env.domaindata["zephyr"]["boards"] = board_catalog["boards"]
+
+    # Preserve existing docnames when reloading the catalog
+    existing_boards = app.env.domaindata.get("zephyr", {}).get("boards", {})
+    new_boards = board_catalog["boards"]
+    for board_name, board_data in new_boards.items():
+        if board_name in existing_boards and "docname" in existing_boards[board_name]:
+            board_data["docname"] = existing_boards[board_name]["docname"]
+
+    app.env.domaindata["zephyr"]["boards"] = new_boards
     app.env.domaindata["zephyr"]["shields"] = board_catalog["shields"]
     app.env.domaindata["zephyr"]["vendors"] = board_catalog["vendors"]
     app.env.domaindata["zephyr"]["socs"] = board_catalog["socs"]