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

services/context/org.go

@@ -260,8 +260,7 @@ func HandleOrgAssignment(ctx *Context, args ...bool) {
 	ctx.Data["IsFollowing"] = ctx.Doer != nil && user_model.IsFollowing(ctx, ctx.Doer.ID, ctx.ContextUser.ID)
 	if len(ctx.ContextUser.Description) != 0 {
 		content, err := markdown.RenderString(&markup.RenderContext{
-			Metas: map[string]string{"mode": "document"},
-			Ctx:   ctx,
+			Ctx: ctx,
 		}, ctx.ContextUser.Description)
 		if err != nil {
 			ctx.ServerError("RenderString", err)