DEVDOCS-6611 - Fastlane integration guide for merchants with custom checkout (#1209)

Created guide article from original PayPal team Google Doc

#
[DEVDOCS-6611](https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6611)


## What changed?

* The Dev Center now includes a guide to integrating Fastlane by PayPal
in older versions of Open Checkout.

## Release notes draft

* We've created a guide to integrating Fastlane by PayPal with your
custom checkout. This provides requirements and setup steps for
configuring Fastlane in earlier versions of checkout-js.

## Anything else?

ping @bc-terra @nkostohryz 


[DEVDOCS-6611]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6611?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: bc-Vince