FIX: Workaround header dropdown rendering issue (#1298)

* FIX: explicitly set the number of items that should appear in the header.

This suppresses a dropdown from kicking in after 5 items, effectively creating a dropdown with only 1 item.
Instead, this just shows all 6 items in the header bar.

Closes #1295 , which raised an issue with the rendering of the dropdown. If a dropdown is ever introduced again, this issue may rear its head again.

* FIX: html_theme_options already defined

doc/conf.py

@@ -145,6 +145,7 @@
     "use_edit_page_button": False,
     "navigation_with_keys": False,
     "show_toc_level": 1,
+    "header_links_before_dropdown": 6,
     "navbar_end": ["theme-switcher", "version-switcher", "navbar-icon-links"],
     "analytics": dict(google_analytics_id="G-C8SH9E98QC"),
     "switcher": {