gh-36799: Create index-pdf.html in html build
    
When we `make doc-pdf`, `reference/index-pdf.html` is also built. With
this PR, it  is built by default when we `make doc-html`.

Here it is: https://deploy-preview-36799--sagemath-
tobias.netlify.app/html/en/reference/index-pdf

This is to fix the issue
#36730 (comment)

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URL: #36799
Reported by: Kwankyu Lee
Reviewer(s):

Author: Release Manager

src/sage_docbuild/builders.py

@@ -654,11 +654,11 @@ def _output_dir(self, type, lang=None):
         os.makedirs(d, exist_ok=True)
         return d
 
-    def pdf(self):
+    def html(self):
         """
-        Build top-level document.
+        Build the top-level document.
         """
-        super().pdf()
+        super().html()
 
         # We want to build master index file which lists all of the PDF file.
         # We modify the file index.html from the "reference_top" target, if it
@@ -668,12 +668,8 @@ def pdf(self):
         reference_dir = os.path.join(SAGE_DOC, 'html', 'en', 'reference')
         output_dir = self._output_dir('html')
 
-        # Check if the top reference index.html exists.
-        try:
-            with open(os.path.join(reference_dir, 'index.html')) as f:
-                html = f.read()
-        except FileNotFoundError:
-            return
+        with open(os.path.join(reference_dir, 'index.html')) as f:
+            html = f.read()
 
         # Install in output_dir a symlink to the directory containing static files.
         # Prefer relative path for symlinks.