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

routers/web/feed/convert.go

@@ -56,7 +56,6 @@ func renderMarkdown(ctx *context.Context, act *activities_model.Action, content
 		Links: markup.Links{
 			Base: act.GetRepoLink(ctx),
 		},
-		Type: markdown.MarkupName,
 		Metas: map[string]string{
 			"user": act.GetRepoUserName(ctx),
 			"repo": act.GetRepoName(ctx),