DEVDOCS-6355 - added pre-render check out pages info to stencil overview (#1056)

<!-- Ticket number or summary of work -->
# [DEVDOCS-6355]


## What changed?
<!-- Provide a bulleted list in the present tense -->
* All stencil themes will now utilize the speculation api for
pre-rendered checkout pages
* This PR updates the documentation to note that it is utilized with
limitations

## Release notes draft
Updated stencil documentation to include pre-rendered checkout pages
using the speculation API

ping @bc-terra 

[DEVDOCS-6355]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6355?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

---------

Co-authored-by: bc-Vince <[email protected]>

Author: chris-nowicki

docs/storefront/stencil/start/index.mdx

@@ -52,6 +52,27 @@ Stencil's use of **YAML Front Matter** allows developers to request only the obj
 
 Stencil themes can access remote objects through event hook and use the hooks to trigger defined events based on shopper behavior. This will allow you to collect product data and optimize a shopper's experience. To facilitate theme-building, BigCommerce provides [stencil-utils](/docs/storefront/stencil/utils/reference) -- a client-side library for managing event hooks.
 
+## Pre-rendered Checkout pages
+
+All stencil themes utilize the [Speculation Rules API](https://developer.mozilla.org/en-US/docs/Web/API/Speculation_Rules_API) to pre-render checkout pages, improving performance and user experience.
+
+**Pre-rendering** means the browser begins loading a page in the background (for example, when a shopper hovers over or is likely to click a checkout link) so that it appears almost instantly when opened.
+
+### Limitations
+
+- Currently only works for non-custom checkout pages.
+- Currently only works if the shopper is using Google Chrome and Edge based browsers.  Check [browser compatibility](https://developer.mozilla.org/en-US/docs/Web/API/Speculation_Rules_API#browser_compatibility) to see latest browser support.
+- Does not work on iOS.
+- The checkout won't pre-render if the shopper updates their cart on the cart page.
+
+<Callout type="info">
+  If you are hosting your own analytics you'll need to make changes to your implementation to ensure compatibility.  See our [hosted analytics documentation](https://developer.bigcommerce.com/docs/integrations/hosted-analytics) for more information.
+</Callout>
+
+<Callout type="info">
+  Custom checkout pages are not supported.  This includes the B2B Edition checkout and any custom checkout pages built with the [Checkout SDK](https://github.com/bigcommerce/checkout-sdk-js) or forked from [Open Checkout](https://github.com/bigcommerce/checkout-js).
+</Callout>
+
 ## Blueprint (Legacy framework)