fix: Make the new Stripe documentation link a shortlink

[Benunc]

Bug Report

User Story

As Matt Cromwell, I want to be able to update the shortlink for Stripe webhook functionality to direct to the new (yet to be written) primer on why the webhook is necessary even for one-time donations so that users don't get confused by the recurring-focus of the doc that is currently linked.

Current Behavior

It is hard-coded to https://givewp.com/documentation/add-ons/recurring-donations/supported-payment-gateways/stripe/%20

Expected Behavior

It should be linked to docs.givewp.com/stripe-free-webhook (or similar)

Bug Type

• [x] This bug describes functionality that never worked as expected.

Steps to Reproduce

1. Navigate to Donations > Settings > Payment Gateways > Stripe settings
2. See the link for docuementation under the webhook setting
3. It's hard-coded.

Visuals

Possible Solution

change the link here https://github.com/impress-org/give/blob/2c3ef8f73ac6839cee0cb6ac2a7f1c9e780ae5c7/includes/gateways/stripe/includes/admin/class-give-stripe-admin-settings.php#L540

Related

https://secure.helpscout.net/conversation/886715028/42324/ (he got confused when I sent him to the recurring doc)

Acceptance Criteria

• [ ] the link goes to a shortlink approved by @mathetos

[Benunc]

Marked high priority, because with the new docs restructure for Stripe/stripe premium/recurring, the page it goes to now doesn't even have the webhook directions on it. We need it to be routed to the free stripe docs.