cloud: update Premium doc links and add Features to TOC (#22827) (#22828)

Author: ti-chi-bot

TOC-tidb-cloud-premium.md

@@ -6,9 +6,10 @@
 ## GET STARTED
 
 - Why TiDB Cloud
-  - [TiDB Cloud Introduction](/tidb-cloud/tidb-cloud-intro.md)
+  - [Introduction](/tidb-cloud/tidb-cloud-intro.md)
+  - [Features](/tidb-cloud/features.md)
   - [MySQL Compatibility](/mysql-compatibility.md)
-- Get Started with TiDB Cloud
+- Get Started
   - [Try Out TiDB Cloud](/tidb-cloud/tidb-cloud-quickstart.md)
   - [Try Out HTAP](/tidb-cloud/tidb-cloud-htap-quickstart.md)
 - Key Concepts

system-variables.md

@@ -1659,7 +1659,7 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 
 > **Note:**
 >
-> This variable is read-only for [{{{ .premium }}}](https://docs-preview.pingcap.com/tidbcloud/tidb-cloud-intro/#deployment-options).
+> This variable is read-only for [{{{ .premium }}}](https://docs.pingcap.com/tidbcloud/select-cluster-tier/#premium).
 
 </CustomContent>
 
@@ -5682,7 +5682,7 @@ SHOW WARNINGS;
 
 > **Note:**
 >
-> This variable is read-only for [{{{ .premium }}}](https://docs-preview.pingcap.com/tidbcloud/tidb-cloud-intro/#deployment-options).
+> This variable is read-only for [{{{ .premium }}}](https://docs.pingcap.com/tidbcloud/select-cluster-tier/#premium).
 
 </CustomContent>