Split parse and render for Goldmark

This also speeds up situations where you only need the fragments/toc and not the rendered content, e.g. Related
with fragments type indexing:

```bash

name            old time/op    new time/op    delta
RelatedSite-10    12.3ms ± 2%    10.7ms ± 1%  -12.95%  (p=0.029 n=4+4)

name            old alloc/op   new alloc/op   delta
RelatedSite-10    38.6MB ± 0%    38.2MB ± 0%   -1.08%  (p=0.029 n=4+4)

name            old allocs/op  new allocs/op  delta
RelatedSite-10      117k ± 0%      115k ± 0%   -1.36%  (p=0.029 n=4+4)
```

Fixes #10750

markup/converter/converter.go

@@ -74,7 +74,7 @@ var NopConverter = new(nopConverter)
 
 type nopConverter int
 
-func (nopConverter) Convert(ctx RenderContext) (Result, error) {
+func (nopConverter) Convert(ctx RenderContext) (ResultRender, error) {
 	return &bytes.Buffer{}, nil
 }
 
@@ -85,15 +85,29 @@ func (nopConverter) Supports(feature identity.Identity) bool {
 // Converter wraps the Convert method that converts some markup into
 // another format, e.g. Markdown to HTML.
 type Converter interface {
-	Convert(ctx RenderContext) (Result, error)
+	Convert(ctx RenderContext) (ResultRender, error)
 	Supports(feature identity.Identity) bool
 }
 
-// Result represents the minimum returned from Convert.
-type Result interface {
+// ParseRenderer is an optional interface.
+// The Goldmark converter implements this, and this allows us
+// to extract the ToC without having to render the content.
+type ParseRenderer interface {
+	Parse(RenderContext) (ResultParse, error)
+	Render(RenderContext, any) (ResultRender, error)
+}
+
+// ResultRender represents the minimum returned from Convert and Render.
+type ResultRender interface {
 	Bytes() []byte
 }
 
+// ResultParse represents the minimum returned from Parse.
+type ResultParse interface {
+	Doc() any
+	TableOfContents() *tableofcontents.Fragments
+}
+
 // DocumentInfo holds additional information provided by some converters.
 type DocumentInfo interface {
 	AnchorSuffix() string