feat: pdfHeaderTemplate/pdfFooterTemplate supports value as file path

docs/docs/pdf.md

@@ -85,7 +85,7 @@ Indicates whether to include background graphics when rendering the pdf.
 
 ### `pdfHeaderTemplate`
 
-HTML template for the print header. Should be valid HTML markup with following HTML elements used to inject printing values into them:
+HTML template for the print header, or a path to an HTML page relative to the root of the output directory. Should be valid HTML markup with following HTML elements used to inject printing values into them:
 
 - `<span class='pageNumber'></span>`: current page number.
 - `<span class='totalPages'></span>`: total pages in the document.
@@ -95,7 +95,7 @@ HTML template for the print header. Should be valid HTML markup with following H
 
 ### `pdfFooterTemplate`
 
-HTML template for the print footer. Should use the same format as the [header template](#pdfheadertemplate). Uses the following default footer template if unspecified:
+HTML template for the print footer, or a path to an HTML page relative to the root of the output directory. Should use the same format as the [header template](#pdfheadertemplate). Uses the following default footer template if unspecified:
 
 ```html
 <div style="width: 100%; font-size: 12px;">

src/Docfx.App/PdfBuilder.cs

@@ -186,8 +186,8 @@ IResult TocPage(string url)
 
         Task<byte[]> PrintHeaderFooter(Outline toc, int pageNumber, int totalPages)
         {
-            var headerTemplate = ExpandTemplate(toc.pdfHeaderTemplate, pageNumber, totalPages);
-            var footerTemplate = ExpandTemplate(toc.pdfFooterTemplate ?? DefaultFooterTemplate, pageNumber, totalPages);
+            var headerTemplate = ExpandTemplate(GetHeaderFooter(toc.pdfHeaderTemplate), pageNumber, totalPages);
+            var footerTemplate = ExpandTemplate(GetHeaderFooter(toc.pdfFooterTemplate) ?? DefaultFooterTemplate, pageNumber, totalPages);
 
             return headerFooterCache.GetOrAdd((headerTemplate, footerTemplate), _ => PrintHeaderFooterCore());
 
@@ -222,6 +222,22 @@ static string ExpandTemplate(string? pdfTemplate, int pageNumber, int totalPages
                     .Replace("<span class='totalPages'></span>", $"<span>{totalPages}</span>")
                     .Replace("<span class=\"totalPages\"></span>", $"<span>{totalPages}</span>");
             }
+
+            string? GetHeaderFooter(string? template)
+            {
+                if (string.IsNullOrEmpty(template))
+                    return template;
+
+                try
+                {
+                    var path = Path.Combine(outputFolder, template);
+                    return File.Exists(path) ? File.ReadAllText(path) : template;
+                }
+                catch
+                {
+                    return template;
+                }
+            }
         }
     }