Fix the issue ref rendering for wiki (go-gitea#28556)

Fix go-gitea#28526, regression of 
* go-gitea#26365

(although the author of go-gitea#26365 has recent activities, but there is no
response for the regression, so I proposed this quick fix and keep the
fix simple to make it easier to backport to 1.21)

Author: wxiaoguang

modules/markup/html.go

@@ -852,7 +852,9 @@ func fullIssuePatternProcessor(ctx *RenderContext, node *html.Node) {
 }
 
 func issueIndexPatternProcessor(ctx *RenderContext, node *html.Node) {
-	if ctx.Metas == nil || ctx.Metas["mode"] == "document" {
+	// FIXME: the use of "mode" is quite dirty and hacky, for example: what is a "document"? how should it be rendered?
+	// The "mode" approach should be refactored to some other more clear&reliable way.
+	if ctx.Metas == nil || (ctx.Metas["mode"] == "document" && !ctx.IsWiki) {
 		return
 	}
 	var (