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/markdown/markdown.go

@@ -257,9 +257,7 @@ func (Renderer) Render(ctx *markup.RenderContext, input io.Reader, output io.Wri
 
 // Render renders Markdown to HTML with all specific handling stuff.
 func Render(ctx *markup.RenderContext, input io.Reader, output io.Writer) error {
-	if ctx.Type == "" {
-		ctx.Type = MarkupName
-	}
+	ctx.MarkupType = MarkupName
 	return markup.Render(ctx, input, output)
 }