docs: fix broken mdc: links in Next steps sections

Replace Cursor mdc:docs/ and mdc:fern/ link prefixes with proper
relative slug links across call-concurrency, call-queue-management,
and arguments-encryption. All targets verified against real page
slugs; API request tools now deep-links to /tools/default-tools#api-request.

Author: stephenvapiai

fern/calls/call-concurrency.mdx

@@ -148,6 +148,6 @@ Adjust the `timeRange.step` to inspect usage by hour, day, or week. Peaks that a
 
 ## Next steps
 
-- **[Call queue management](mdc:docs/calls/call-queue-management):** Build a Twilio queue to buffer calls when you hit concurrency caps.
-- **[Outbound campaign planning](mdc:docs/outbound-campaigns/overview):** Design outbound strategies that pair batching with analytics.
-- **[Enterprise plans](mdc:docs/enterprise/plans):** Review larger plans that include higher default concurrency.
+- **[Call queue management](/calls/call-queue-management):** Build a Twilio queue to buffer calls when you hit concurrency caps.
+- **[Outbound campaign planning](/outbound-campaigns/overview):** Design outbound strategies that pair batching with analytics.
+- **[Enterprise plans](/enterprise/plans):** Review larger plans that include higher default concurrency.

fern/calls/call-queue-management.mdx

@@ -701,10 +701,10 @@ Redis is required for this implementation. Ensure your Redis instance is properl
 
 Now that you have a production-ready call queue system with Redis persistence and callback-driven processing:
 
-- **[Advanced Call Features](mdc:docs/calls/call-features):** Explore call recording, analysis, and advanced routing options
-- **[Monitoring & Analytics](mdc:docs/assistants/call-analysis):** Set up comprehensive call analytics and performance monitoring
-- **[Scaling Considerations](mdc:docs/enterprise/plans):** Learn about enterprise features for high-volume deployments
-- **[Assistant Optimization](mdc:docs/assistants/personalization):** Enhance your assistants with personalization and dynamic variables
+- **[Advanced Call Features](/calls/call-features):** Explore call recording, analysis, and advanced routing options
+- **[Monitoring & Analytics](/assistants/call-analysis):** Set up comprehensive call analytics and performance monitoring
+- **[Scaling Considerations](/enterprise/plans):** Learn about enterprise features for high-volume deployments
+- **[Assistant Optimization](/assistants/personalization):** Enhance your assistants with personalization and dynamic variables
 
 <Tip>
 Consider implementing health checks, metrics collection, and alerting around your Redis counters and queue processing latency for production monitoring.

fern/tools/arguments-encryption.mdx

@@ -213,6 +213,6 @@ Follow these guidelines to maintain secure encryption:
 
 Now that you have tool argument encryption configured:
 
-- **[Custom tools](mdc:fern/tools/custom-tools):** Learn more about creating custom tools
-- **[API request tools](mdc:fern/tools/api-request):** Configure API request tools with encrypted arguments
-- **[Server URLs](mdc:fern/server-url):** Set up secure server endpoints for receiving encrypted data
+- **[Custom tools](/tools/custom-tools):** Learn more about creating custom tools
+- **[API request tools](/tools/default-tools#api-request):** Configure API request tools with encrypted arguments
+- **[Server URLs](/server-url):** Set up secure server endpoints for receiving encrypted data