Refactor render system (#32492)

There were too many patches to the Render system, it's really difficult
to make further improvements.

This PR clears the legacy problems and fix TODOs.

1. Rename `RenderContext.Type` to `RenderContext.MarkupType` to clarify
its usage.
2. Use `ContentMode` to replace `meta["mode"]` and `IsWiki`, to clarify
the rendering behaviors.
3. Use "wiki" mode instead of "mode=gfm + wiki=true"
4. Merge `renderByType` and `renderByFile`
5. Add more comments

----

The problem of "mode=document": in many cases it is not set, so many
non-comment places use comment's hard line break incorrectly

modules/markup/html.go

@@ -442,12 +442,11 @@ func createLink(href, content, class string) *html.Node {
 	a := &html.Node{
 		Type: html.ElementNode,
 		Data: atom.A.String(),
-		Attr: []html.Attribute{
-			{Key: "href", Val: href},
-			{Key: "data-markdown-generated-content"},
-		},
+		Attr: []html.Attribute{{Key: "href", Val: href}},
+	}
+	if !RenderBehaviorForTesting.DisableInternalAttributes {
+		a.Attr = append(a.Attr, html.Attribute{Key: "data-markdown-generated-content"})
 	}
-
 	if class != "" {
 		a.Attr = append(a.Attr, html.Attribute{Key: "class", Val: class})
 	}