docs: Document subscription payment failure and cancellation process

[rishi-raj-jain]

Document the subscription payment failure handling process including:

Grace period for "Past Due" subscriptions (21 days)

Payment retry schedule (4 retries)

This documentation should be added to help customer support and users understand the subscription lifecycle when payments for renewals fail.

Context: Customer inquiry about how long the grace period is before subscription deletion events are triggered.

[rishi-raj-jain]

After 21 days the subscription will be moved to cancel. We will have 4 payment retries: https://github.com/polarsource/polar/blob/main/server/polar/config.py#L289