Added docs on avoiding duplicate HTTP spans when using OTEL (#15378)

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## DESCRIBE YOUR PR

Resolves getsentry/sentry-dotnet#4670

## IS YOUR CHANGE URGENT?  

- [ ] None: Not urgent, can wait up to 1 week+

## SLA

cc: @Flash0ver 

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

Co-authored-by: Michi Hoffmann <cleptric@users.noreply.github.com>

Author: jamescrosswell

platform-includes/performance/opentelemetry-setup/dotnet.aspnetcore.mdx

@@ -21,5 +21,10 @@ builder.WebHost.UseSentry(options =>
     options.SendDefaultPii = true;
     options.TracesSampleRate = 1.0;
     options.UseOpenTelemetry(); // <-- Configure Sentry to use OpenTelemetry trace information
+    options.DisableSentryHttpMessageHandler = true; // <-- Disable Sentry's HttpClient instrumentation to avoid duplicate spans
 });
 ```
+
+<Alert level="info" title="Important Note">
+When using OpenTelemetry to instrument HttpClient spans via `AddHttpClientInstrumentation`, you **must** set `DisableSentryHttpMessageHandler = true` in the Sentry options to disable Sentry's built-in HttpClient instrumentation. This prevents the creation of duplicate spans for HTTP requests.
+</Alert>