docs: create a guide on how to limit trials per user based on their past subscriptions

[rishi-raj-jain]

trafficstars

From the discord chat

Create a guide on:

• have two variants of each product, one with trial and the other without trial (can label one of them to say "With Trial")
• create an API route where in the backend logic sees the customer/user that's logged in and makes the get customer state API call to see their active subscriptions OR the List Subscriptions API call to see all the subscriptions via their External ID
• take them to the desired product (i.e. with or without trial) via creating a checkout session via the API