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/orgmode/orgmode.go

@@ -144,14 +144,15 @@ func (r *Writer) resolveLink(kind, link string) string {
 		}
 
 		base := r.Ctx.Links.Base
-		if r.Ctx.IsWiki {
+		isWiki := r.Ctx.ContentMode == markup.RenderContentAsWiki
+		if isWiki {
 			base = r.Ctx.Links.WikiLink()
 		} else if r.Ctx.Links.HasBranchInfo() {
 			base = r.Ctx.Links.SrcLink()
 		}
 
 		if kind == "image" || kind == "video" {
-			base = r.Ctx.Links.ResolveMediaLink(r.Ctx.IsWiki)
+			base = r.Ctx.Links.ResolveMediaLink(isWiki)
 		}
 
 		link = util.URLJoin(base, link)