docs(mcp): document MCP server variables and credential store

CLAUDE.md

@@ -0,0 +1,7 @@
+When making public-facing docs, PR descriptions, comments, issues, commit messages, etc., always follow these guidelines to sound less AI-y:
+
+- prefer not to use emojis
+- don't use "—". Instead, reach for ";", ".", etc.
+- don't use the pattern "It's not X, it's Y", "You're not X, you're Y", etc.
+- don't use bulleted or numbered lists unless it would be nonsensical not to. Instead, prefer prose
+- be information dense, concise, and clear

docs/mcp.md

@@ -166,6 +166,16 @@ These headers get sent with every request to the server. That's it.
 - You want full control over exactly what headers are sent
 - You're debugging and need to quickly add headers without changing auth configuration
 
+### Server Variables
+
+Store credentials on the server and reference them in static headers or authentication with `${VAR_NAME}` (e.g. `${DB_PROTOCOL}://${CORP_USERNAME}:${CORP_PASSWORD}@${DB_HOSTNAME}`). Scope each variable as **Instance** (shared) or **Per-user** (each user supplies their own).
+
+<iframe width="840" height="500" src="https://www.loom.com/embed/12878e2be19140069170c3a270b50d1c" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>
+
+**When to use this:**
+- Each user needs to connect with their own static credentials
+- You want to reuse certain shared details (e.g., DB url) across users
+
 </TabItem>
 
 <TabItem value="config" label="config.yaml">

release_notes/v1.89.0/index.md

@@ -64,6 +64,8 @@ pip install litellm==1.89.0
 
 This release lets you securely store per-server credentials for MCP servers directly on the gateway. Define variables once on a server, scoped either as **Instance** (shared across all users) or **Per-user** (each user supplies their own value), and reference them in static headers or authentication using `${VAR_NAME}` syntax (for example, `${DB_PROTOCOL}://${CORP_USERNAME}:${CORP_PASSWORD}@${DB_HOSTNAME}`), letting each user connect their own identity.
 
+[Get Started](../../docs/mcp#server-variables)
+
 ## New Providers and Endpoints
 
 ### New Providers (3 new providers)