Support setting number of header links

Now users can set

[sphinx.theme]
header_links_before_dropdown = 4

rather than extending conf.py.

docs/project-guides/guides/toml-reference.rst

@@ -283,6 +283,16 @@ If the Python API is oriented towards contributors, such as in an application or
 
 Configurations related to the Sphinx HTML theme.
 
+header_links_before_dropdown
+----------------------------
+
+|optional|
+
+Number of links to show in the navigation head before folding extra items into a "More" dropdown.
+The default is 5.
+
+If the section titles are long you may need to reduce this number.
+
 .. _guide-project-show-github-edit-link:
 
 show_github_edit_link

src/documenteer/conf/_toml.py

@@ -141,6 +141,14 @@ class ThemeModel(BaseModel):
         True, description="Show a link to edit on GitHub if True"
     )
 
+    header_links_before_dropdown: int = Field(
+        5,
+        description=(
+            "Number of links in the header nav before showing a 'More' "
+            "dropdown."
+        ),
+    )
+
 
 class SphinxModel(BaseModel):
     """Model for Sphinx configurations in documenteer.toml."""
@@ -194,7 +202,7 @@ class SphinxModel(BaseModel):
         ),
     )
 
-    theme: ThemeModel
+    theme: ThemeModel = Field(default_factory=lambda: ThemeModel())
 
     intersphinx: Optional[IntersphinxModel]
 
@@ -463,3 +471,13 @@ def set_edit_on_github(
             "github_version"
         ] = self.conf.project.github_default_branch
         html_context["doc_path"] = doc_dir
+
+    @property
+    def header_links_before_dropdown(self) -> int:
+        """Number of links to show in the nav head before folding extra items
+        into a More dropdown.
+        """
+        if self.conf.sphinx:
+            return self.conf.sphinx.theme.header_links_before_dropdown
+        else:
+            return 5

src/documenteer/conf/guide.py

@@ -210,7 +210,7 @@
 # further.  For a list of options available for each theme, see the
 # documentation.
 html_theme_options = {
-    "header_links_before_dropdown": 5,
+    "header_links_before_dropdown": _conf.header_links_before_dropdown,
     "external_links": [{"name": "Rubin docs", "url": "https://www.lsst.io"}],
     "icon_links": [],
     "logo": {