[docs] Remove pulsar-admin from reference section (apache#9813)

### Motivation

Currently, we have [CLI website](http://pulsar.apache.org/tools/pulsar-admin/2.8.0-SNAPSHOT/), which generate `pulsar-admin` commands and usage automatically.

### Modifications
1. Remove the `pulsar-admin` section from Reference.
2. Navigate the original link to `pulsar-admin` reference page to the CLI website.

Related info:
1. The change will take effect in 2.8.0 release. For the earlier releases, keep it in the sidebar.
2. We still keep the `reference-pulsar-admin.md` file in the directory. If [this link](https://pulsar.apache.org/docs/en/next/pulsar-admin/) is used externally, it still works.
3. In future, if we add reference to the complete `pulsar-admin` usage, we can navigate to the CLI website. And we'll not maintain the `reference-pulsar-admin.md` file anymore.

site2/docs/admin-api-overview.md

@@ -16,7 +16,7 @@ You can interact with the admin interface via:
      $ bin/pulsar-admin
     ```
 
-    For details of `pulsar-admin` tool, see the [Pulsar command-line tools](reference-pulsar-admin.md) doc.
+    For complete commands of `pulsar-admin` tool, see [Pulsar admin snapshot](http://pulsar.apache.org/tools/pulsar-admin/).
 
 
 > **The REST API is the admin interface**. Both the `pulsar-admin` CLI tool and the Java client use the REST API. If you implement your own admin interface client, you should use the REST API. 

site2/website/sidebars.json

@@ -150,7 +150,6 @@
     "Reference": [
       "reference-terminology",
       "reference-cli-tools",
-      "pulsar-admin",
       "reference-configuration",
       "reference-metrics"
     ]