Support switching TiDB Cloud plans while keeping users on the same document #640

Issue description:

Currently, if one documentation topic (page) exist in multiple plans such as (Starter, Essential, Dedicated): assume users open https://docs.pingcap.com/tidbcloud/delete-tidb-cluster/?plan=essential, then they switch the plan to Dedicated by clicking the plan box in the upper left corner, they'll brought to the landing page of Dedicated docs (the default plan shown: https://docs.pingcap.com/tidbcloud/).

However, it would be better to guide users to the same topic page: https://docs.pingcap.com/tidbcloud/delete-tidb-cluster/?plan=dedicated

Solution:

• For the topic page that is used by two or more plans: support guiding users to the corresponding page of the target plan documentation.
• For the topic page that is used by only one single plan: guiding users to the landing page of the target plan documentation.