DEVDOCS-6388 | Feedback - How to see which version of Checkout SDK yo… (#953)

…ur custom checkout is using

<!-- Ticket number or summary of work -->
# [DEVDOCS-6388]


## What changed?
<!-- Provide a bulleted list in the present tense -->
* Adding a FAQs section and 1 FAQ for how to find the Checkout SDK
version.

## Release notes draft
<!-- Provide an entry for the release notes using simple, conversational
language. Don't be too technical. Explain how the change will benefit
the merchant and link to the feature.

Examples:
* The newly-released [X feature] is now available to use. Now, you’ll be
able to [perform Y action].
* We're happy to announce [X feature], which can help you [perform Y
action].
* [X feature] helps you to create [Y response] using the [Z query
parameter]. Now, you can deliver [ex, localized shopping experiences for
your customers].
* Fixed a bug in the [X endpoint]. Now the [Y field] will appear when
you click [Z option]. -->
* Added instructions for finding the Checkout SDK version for an
existing custom checkout.

## Anything else?
<!-- Add related PRs, salient notes, additional ticket numbers, etc. -->

@bc-terra - ready for review

[DEVDOCS-6388]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6388?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: bc-burney

docs/storefront/cart-checkout/checkout-sdk/index.mdx

@@ -171,6 +171,17 @@ In the developer console, you can see the entire available cart object. The outp
 We can, for example, get or set shipping and billing addresses, sign in customers, apply coupons or gift certificates, initialize payment, or complete orders.
 </Callout>
 
+## FAQs
+
+**How can I find the Checkout SDK version my custom checkout is currently using?**
+
+The Checkout SDK version is included in all API request headers from the checkout.
+
+To find your version:
+
+1. Open Developer Tools in your browser.  
+2. Load your checkout page.  
+3. Under the Network tab in Developer Tools, search for `x-checkout-sdk-version`.
 
 ## Resources