[docs] Remove Dashboard from sidebar (apache#6097)

### Motivation
Pulsar dashboard is deprecated. We need to remove it from sidebar.

### Modifications
1. Remove Pulsar dashboard from the sidebar (for both master and 2.5.0 release);
2. Add related note and link to Pulsar dashboard in Pulsar manager.

site2/docs/administration-pulsar-manager.md

@@ -6,6 +6,9 @@ sidebar_label: Pulsar Manager
 
 Pulsar Manager is a web-based GUI management and monitoring tool that helps administrators and users manage and monitor tenants, namespaces, topics, subscriptions, brokers, clusters, and so on, and supports dynamic configuration of multiple environments.
 
+> Note   
+> If you monitor your current stats with [Pulsar dashboard](administration-dashboard.md), you can try to use Pulsar Manager instead. Pulsar dashboard is deprecated.
+
 ## Install
 
 The easiest way to use the Pulsar Manager is to run it inside a [Docker](https://www.docker.com/products/docker) container.

site2/website/sidebars.json

@@ -64,7 +64,6 @@
     "Administration": [
       "administration-zk-bk",
       "administration-geo",
-      "administration-dashboard",
       "administration-pulsar-manager",
       "administration-stats",
       "administration-load-balance",

site2/website/versioned_docs/version-2.5.0/administration-pulsar-manager.md

@@ -7,6 +7,9 @@ original_id: administration-pulsar-manager
 
 Pulsar Manager is a web-based GUI management and monitoring tool that helps administrators and users manage and monitor tenants, namespaces, topics, subscriptions, brokers, clusters, and so on, and supports dynamic configuration of multiple environments.
 
+> Note   
+> If you monitor your current stats with [Pulsar dashboard](administration-dashboard.md), you can try to use Pulsar Manager instead. Pulsar dashboard is deprecated.
+
 ## Install
 
 The easiest way to use the Pulsar Manager is to run it inside a [Docker](https://www.docker.com/products/docker) container.

site2/website/versioned_sidebars/version-2.5.0-sidebars.json

@@ -64,7 +64,6 @@
     "Administration": [
       "version-2.5.0-administration-zk-bk",
       "version-2.5.0-administration-geo",
-      "version-2.5.0-administration-dashboard",
       "version-2.5.0-administration-pulsar-manager",
       "version-2.5.0-administration-stats",
       "version-2.5.0-administration-load-balance",