Correctly specify the source path to a logo to copy_asset_file (#1746)

I think this fixes #1385, I think that bug report is expecting to be able to pass a logo in the docs/_static directory but according to the docs here: https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/branding.html#different-logos-for-light-and-dark-mode the path should be relative to conf.py.

I think the actual bug is that we were not passing the correct source to the sphinx copy function which exits silently (for some reason) if the source does not exist.

src/pydata_sphinx_theme/logo.py

@@ -63,6 +63,7 @@ def copy_logo_images(app: Sphinx, exception=None) -> None:
     warning = partial(maybe_warn, app)
     logo = get_theme_options_dict(app).get("logo", {})
     staticdir = Path(app.builder.outdir) / "_static"
+    assert staticdir.is_absolute()
     for kind in ["light", "dark"]:
         path_image = logo.get(f"image_{kind}")
         if not path_image or isurl(path_image):
@@ -71,12 +72,14 @@ def copy_logo_images(app: Sphinx, exception=None) -> None:
             # file already exists in static dir e.g. because a theme has
             # bundled the logo and installed it there
             continue
-        if not (Path(app.srcdir) / path_image).exists():
+        full_logo_path = Path(app.srcdir) / path_image
+        assert full_logo_path.is_absolute()
+        if not full_logo_path.exists():
             warning(f"Path to {kind} image logo does not exist: {path_image}")
         # Ensure templates cannot be passed for logo path to avoid security vulnerability
         if path_image.lower().endswith("_t"):
             raise ExtensionError(
                 f"The {kind} logo path '{path_image}' looks like a Sphinx template; "
                 "please provide a static logo image."
             )
-        copy_asset_file(path_image, staticdir)
+        copy_asset_file(str(full_logo_path), staticdir)