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/transform_image.go

@@ -21,7 +21,7 @@ func (g *ASTTransformer) transformImage(ctx *markup.RenderContext, v *ast.Image)
 	// Check if the destination is a real link
 	if len(v.Destination) > 0 && !markup.IsFullURLBytes(v.Destination) {
 		v.Destination = []byte(giteautil.URLJoin(
-			ctx.Links.ResolveMediaLink(ctx.IsWiki),
+			ctx.Links.ResolveMediaLink(ctx.ContentMode == markup.RenderContentAsWiki),
 			strings.TrimLeft(string(v.Destination), "/"),
 		))
 	}