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/extensions/developing-extensions.md

@@ -8,7 +8,7 @@ Extensions can add the following capabilities to Zed:
 - [Themes](./themes.md)
 - [Icon Themes](./icon-themes.md)
 - [Slash Commands](./slash-commands.md)
-- [Context Servers](./context-servers.md)
+- [MCP Servers](./mcp-extensions.md)
 
 ## Developing an Extension Locally