markup/tableofcontents: Add pre/post config options

Add Pre and Post configuration options to customize the HTML wrapper
around the table of contents. This allows users to add custom elements
like headings for accessibility.

Example config:
[markup.tableOfContents]
  pre = '<nav id="TableOfContents"><h2>Contents</h2>'
  post = '</nav>'

The pre/post parameters are optional in ToHTML() for backward
compatibility - existing templates continue to work without changes.

Fixes #8338

hugolib/page__content.go

@@ -670,6 +670,8 @@ func (c *cachedContentScope) contentToC(ctx context.Context) (contentTableOfCont
 					cfg.TableOfContents.StartLevel,
 					cfg.TableOfContents.EndLevel,
 					cfg.TableOfContents.Ordered,
+					cfg.TableOfContents.Pre,
+					cfg.TableOfContents.Post,
 				)
 				return err
 			}

markup/tableofcontents/tableofcontents.go

@@ -148,7 +148,8 @@ func (toc *Fragments) addAt(h *Heading, row, level int) {
 }
 
 // ToHTML renders the ToC as HTML.
-func (toc *Fragments) ToHTML(startLevel, stopLevel any, ordered bool) (template.HTML, error) {
+// The pre and post parameters are optional and default to the nav wrapper if not provided.
+func (toc *Fragments) ToHTML(startLevel, stopLevel any, ordered bool, prePost ...string) (template.HTML, error) {
 	if toc == nil {
 		return "", nil
 	}
@@ -163,12 +164,24 @@ func (toc *Fragments) ToHTML(startLevel, stopLevel any, ordered bool) (template.
 		return "", fmt.Errorf("stopLevel: %w", err)
 	}
 
+	// Use default pre/post values if not provided
+	pre := DefaultConfig.Pre
+	post := DefaultConfig.Post
+	if len(prePost) > 0 && prePost[0] != "" {
+		pre = prePost[0]
+	}
+	if len(prePost) > 1 && prePost[1] != "" {
+		post = prePost[1]
+	}
+
 	b := &tocBuilder{
 		s:          strings.Builder{},
 		h:          toc.Headings,
 		startLevel: iStartLevel,
 		stopLevel:  iStopLevel,
 		ordered:    ordered,
+		pre:        pre,
+		post:       post,
 	}
 	b.Build()
 	return template.HTML(b.s.String()), nil
@@ -187,16 +200,18 @@ type tocBuilder struct {
 	startLevel int
 	stopLevel  int
 	ordered    bool
+	pre        string
+	post       string
 }
 
 func (b *tocBuilder) Build() {
 	b.writeNav(b.h)
 }
 
 func (b *tocBuilder) writeNav(h Headings) {
-	b.s.WriteString("<nav id=\"TableOfContents\">")
+	b.s.WriteString(b.pre)
 	b.writeHeadings(1, 0, b.h)
-	b.s.WriteString("</nav>")
+	b.s.WriteString(b.post)
 }
 
 func (b *tocBuilder) writeHeadings(level, indent int, h Headings) {
@@ -260,6 +275,8 @@ var DefaultConfig = Config{
 	StartLevel: 2,
 	EndLevel:   3,
 	Ordered:    false,
+	Pre:        `<nav id="TableOfContents">`,
+	Post:       "</nav>",
 }
 
 type Config struct {
@@ -274,4 +291,12 @@ type Config struct {
 
 	// Whether to produce a ordered list or not.
 	Ordered bool
+
+	// Pre is the HTML to insert before the ToC list.
+	// Default is `<nav id="TableOfContents">`.
+	Pre string
+
+	// Post is the HTML to insert after the ToC list.
+	// Default is `</nav>`.
+	Post string
 }