Clarify the role of the router (#223)

Author: Stephen Barlow

docs/source/index.mdx

@@ -10,7 +10,7 @@ import { colors } from 'gatsby-theme-apollo-core';
 
 > ⚠️ **The Apollo Router is in public pre-alpha.** Until it is generally available, do not use it in business-critical graphs. [Learn about release stages.](https://www.apollographql.com/docs/resources/release-stages/#open-source-release-stages)
 
-The **Apollo Router** is a configurable, high-performance **graph router** for a [federated graph](https://www.apollographql.com/docs/federation/):
+The **Apollo Router** is a configurable, high-performance **graph router** for a federated graph. It slots into any existing [Apollo Federation](https://www.apollographql.com/docs/federation/) architecture (v1 or v2), replacing your Node.js gateway that uses the `@apollo/gateway` library:
 
 ```mermaid
 flowchart BT;
@@ -26,7 +26,7 @@ flowchart BT;
   class clients secondary;
 ```
 
-The Apollo Router is [implemented in Rust](https://github.com/apollographql/router), which provides [performance benefits](https://www.apollographql.com/blog/announcement/backend/apollo-router-our-graphql-federation-runtime-in-rust/) over the TypeScript Apollo Gateway.
+ The Apollo Router is [implemented in Rust](https://github.com/apollographql/router), which provides [performance benefits](https://www.apollographql.com/blog/announcement/backend/apollo-router-our-graphql-federation-runtime-in-rust/) over Node.js.
 
 <p>
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