Add changelog for January 20, 2025 (#159)

Author: quantstruct-bot

fern/changelog/2025-01-20.mdx

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+# Workflow Steps, Trieve Knowledge Base Updates, and Concurrent Calls Tracking
+
+1. **Use Workflow Blocks to Simplify Blocks Steps:** You can now compose complicated Blocks steps with smaller, resuable [Workflow blocks](https://api.vapi.ai/api#:~:text=Workflow) that manage conversations and take actions in external systems.
+
+In addition to normal operations inside [Block steps](https://docs.vapi.ai/blocks/steps) - you can now [Say messages](https://api.vapi.ai/api#:~:text=Say), [Gather information](https://api.vapi.ai/api#:~:text=Gather), or connect to other workflow [Edges](https://api.vapi.ai/api#:~:text=Edge) based on a [LLM evaluating a condition](https://api.vapi.ai/api#:~:text=SemanticEdgeCondition), or a more [logic-based condition](https://api.vapi.ai/api#:~:text=ProgrammaticEdgeCondition). Workflows can be used through `Assistant.model["VapiModel"]` to create custom call workflows. 
+
+2. **Trieve Knowledge Base Integration Improvements:** You should now configure [Trieve knowledge bases](https://api.vapi.ai/api#:~:text=TrieveKnowledgeBase) using the new `createPlan` and `searchPlan` fields instead of specifying the raw vector plans directly. The new plans allow you to create or import trieve plans directly, and specify the type of search more precisely than before.
+
+3. **Updated Concurrency Tracking:** Your subscriptions now track active calls with `concurrencyCounter`, replacing `concurrencyLimit`. This does not affect how you reserve concurrent calls through [billing add-ons](https://dashboard.vapi.ai/org/billing/add-ons).
+
+<Frame caption="Concurrent Calls Tracking">
+    <img src="../static/images/changelog/billing-addon-concurrent-calls.png" />
+</Frame>
+
+4. **Define Allowed Values with `type` using `JsonSchema`:** You can restrict model outputs to specific values inside Blocks or tool calls using the new `type` property in [JsonSchema](https://api.vapi.ai/api#:~:text=JsonSchema). Supported types include `string`, `number`, `integer`, `boolean`, `array` (which also needs `items` to be defined), and `object` (which also needs `properties` to be defined).
+