Add non-configurable headers guidance to custom user agent section

- Updated the setting-custom-user-agent partial to advise destination servers to use non-configurable headers for identifying/blocking Browser Rendering requests
- Links to the non-configurable headers section of the automatic request headers reference
- Propagates to all REST API endpoint pages (content, screenshot, pdf, markdown, scrape, links, snapshot, json, crawl)

Author: kathayl

src/content/partials/browser-rendering/setting-custom-user-agent.mdx

@@ -3,5 +3,5 @@
 You can change the user agent at the page level by passing `userAgent` as a top-level parameter in the JSON body. This is useful if the target website serves different content based on the user agent.
 
 :::note
-The `userAgent` parameter does not bypass bot protection. Requests from Browser Rendering will always be identified as a bot. 
+The `userAgent` parameter does not bypass bot protection. Requests from Browser Rendering will always be identified as a bot. Because the User-Agent is configurable, destination servers looking to identify or block Browser Rendering requests should use the [non-configurable headers](/browser-rendering/reference/automatic-request-headers/#non-configurable-headers) rather than relying on the User-Agent string.
 :::