docs: Improve MCP-related pages (#32422)

While creating a new MCP extension this weekend, I visited these pages
and it felt like they could be improved a little bit. I'm renaming the
MCP-related page under the /extension directory to use the "MCP"
acronym, instead of "context servers".

Release Notes:

- N/A

docs/src/SUMMARY.md

@@ -66,7 +66,7 @@
 - [Theme Extensions](./extensions/themes.md)
 - [Icon Theme Extensions](./extensions/icon-themes.md)
 - [Slash Command Extensions](./extensions/slash-commands.md)
-- [Context Server Extensions](./extensions/context-servers.md)
+- [MCP Server Extensions](./extensions/mcp-extensions.md)
 
 # Language Support