docs: show second level in left sidebar (#221)

* docs: add numpy to intersphinx inventory

docs/conf.py

@@ -199,6 +199,7 @@ def fetch_logo(url: str, output_path: str) -> None:
         "thebe": True,
         "thebelab": True,
     },
+    "show_navbar_depth": 2,
     "theme_dev_mode": True,
 }
 html_title = "AmpForm"
@@ -254,6 +255,19 @@ def get_version(package_name: str) -> str:
     return "stable"
 
 
+def get_minor_version(package_name: str) -> str:
+    installed_version = get_version(package_name)
+    if installed_version == "stable":
+        return installed_version
+    matches = re.match(r"^([0-9]+\.[0-9]+).*$", installed_version)
+    if matches is None:
+        raise ValueError(
+            "Could not find documentation for"
+            f" {package_name} v{installed_version}"
+        )
+    return matches[1]
+
+
 intersphinx_mapping = {
     "attrs": (f"https://www.attrs.org/en/{get_version('attrs')}", None),
     "compwa-org": ("https://compwa-org.readthedocs.io", None),
@@ -270,6 +284,7 @@ def get_version(package_name: str) -> str:
         f"https://mpl-interactions.readthedocs.io/en/{get_version('mpl-interactions')}",
         None,
     ),
+    "numpy": (f"https://numpy.org/doc/{get_minor_version('numpy')}", None),
     "pwa": ("https://pwa.readthedocs.io", None),
     "python": ("https://docs.python.org/3", None),
     "qrules": (