Add Conversions API + Parameter Builder integration sections to READMEs

Summary:
Document the Conversions API (CAPI) and the CAPI Parameter Builder integration
in all five Business SDK READMEs (PHP, NodeJS, Java, Python, Ruby).

The Java/NodeJS/Python/Ruby release READMEs are codegen-generated from
codegen/templates/<lang>/README.md, so the new content is added to BOTH the
templates and the generated release files (kept in sync so codegen.sh is a
no-op). The PHP release README is hand-maintained (no template) and edited
directly.

Each README gains:
- A "Conversions API" section: a basic end-to-end server-side event send
  (UserData + CustomData + Event + EventRequest.execute), plus a link to the
  advanced features guide (async, concurrent batching, custom HTTP service).
- A "Conversions API Parameter Builder integration" subsection: setRequestContext
  / set_request_context auto-fills empty event fields from the incoming HTTP
  request (fbc, fbp, event_source_url, referrer_url), the optional Preference
  allowlist, and the non-destructive + order-independent behavior. The
  description of what request object to pass links to the "Framework support"
  section of the matching capi-param-builder GitHub README.

Per-language accuracy matches the shipped code:
- PHP & NodeJS also document client_ip_address auto-fill and automatic PII
  normalization + SHA-256 hashing.
- Java, Python, Ruby note that client_ip_address is not auto-derived yet and
  should still be set manually.

Depends on D107207093

Reviewed By: satwikareddy3, xuanjie22

Differential Revision: D108471483

fbshipit-source-id: f3f64ecbd9107bdd41b3c19e8b347821e87889fe

Author: Jiaming You

README.md

@@ -348,6 +348,63 @@ Here are some hints on troubleshooting:
 * For temporary server issue, typically retry should work after a few seconds.
 * If server persistently responds with "Unknown error," then it is potentially server bug.
 
+## Conversions API
+
+The Business SDK includes a server-side client for the [Conversions API](https://developers.facebook.com/docs/marketing-api/conversions-api), used to send web, app, and offline events to Meta directly from your server.
+
+```java
+import com.facebook.ads.sdk.APIContext;
+import com.facebook.ads.sdk.serverside.*;
+
+APIContext context = new APIContext("<ACCESS_TOKEN>");
+
+UserData userData = new UserData()
+    .email("joe@eg.com")
+    .clientIpAddress(request.getRemoteAddr())
+    .clientUserAgent(request.getHeader("User-Agent"));
+
+CustomData customData = new CustomData()
+    .currency("usd")
+    .value(123.45F);
+
+Event event = new Event()
+    .eventName("Purchase")
+    .eventTime(System.currentTimeMillis() / 1000L)
+    .userData(userData)
+    .customData(customData)
+    .eventSourceUrl("http://jaspers-market.com/product/123")
+    .actionSource(ActionSource.website);
+
+EventRequest eventRequest = new EventRequest("<PIXEL_ID>", context);
+eventRequest.addDataItem(event);
+EventResponse response = eventRequest.execute();
+System.out.println(response);
+```
+
+For advanced features — asynchronous requests, concurrent batching, and a custom HTTP service — see [Meta Business SDK Features for Conversions API](https://developers.facebook.com/documentation/ads-commerce/conversions-api/guides/business-sdk-features/).
+
+### Conversions API Parameter Builder integration
+
+The SDK bundles the [Conversions API Parameter Builder](https://github.com/facebook/capi-param-builder) ([Java README](https://github.com/facebook/capi-param-builder/blob/main/java/README.md)) so it can auto-fill key event parameters straight from the incoming HTTP request. Call `setRequestContext()` on an event and pass the incoming request — see [Framework support](https://github.com/facebook/capi-param-builder/blob/main/java/README.md#framework-support) for exactly what to pass for your framework (e.g. a `javax`/`jakarta` `HttpServletRequest`, a Spring WebFlux `ServerHttpRequest`, or a CGI/WSGI-style `Map`). At send time the SDK runs the Parameter Builder and fills in any of these fields you left empty:
+
+- `user_data.fbc`, `user_data.fbp`
+- `event_source_url`, `referrer_url`
+
+```java
+Event event = new Event()
+    .eventName("Purchase")
+    .eventTime(System.currentTimeMillis() / 1000L)
+    .userData(new UserData().email("joe@eg.com"))
+    .actionSource(ActionSource.website)
+    .setRequestContext(request);
+
+// Optional: gate which fields may be auto-filled (all default true).
+// Order: fbc, fbp, client_ip_address, referrer_url, event_source_url.
+//   .setRequestContext(request, new Preference(true, true, true, true, false));
+```
+
+Auto-fill is **gated** by the optional `Preference` allowlist, **non-destructive** (a value you set yourself is never overwritten), and **order-independent**. Note that in Java, `client_ip_address` is not auto-derived yet — keep setting it (and `client_user_agent`) yourself.
+
 ## SDK Codegen
 Our SDK is autogenerated from [SDK Codegen](https://github.com/facebook/facebook-business-sdk-codegen). If you want to learn more about how our SDK code is generated, please check this repository.