Plugins: Fix plugin query help markdown (#60485)

* refactor(pluginhelp): rewrite as functional component with useAsync

* mimic old behaviour

* feat(pluginhelp): display message if backend returned an empty string

Co-authored-by: Jack Westbrook <jack.westbrook@gmail.com>

pkg/api/plugins.go

@@ -271,7 +271,7 @@ func (hs *HTTPServer) GetPluginMarkdown(c *models.ReqContext) response.Response
 
 	// fallback try readme
 	if len(content) == 0 {
-		content, err = hs.pluginMarkdown(c.Req.Context(), pluginID, "help")
+		content, err = hs.pluginMarkdown(c.Req.Context(), pluginID, "readme")
 		if err != nil {
 			return response.Error(501, "Could not get markdown file", err)
 		}