docs: Add link to MCP extensions in the overview page (#32458)

Danilo Leal and Peter Tripp created

Follow up to https://github.com/zed-industries/zed/pull/32422. Missed
this one in this latest round of MCP-related docs changes.

Release Notes:

- N/A

Co-authored-by: Peter Tripp <peter@zed.dev>

Change summary

docs/src/extensions.md                | 1 +
docs/src/extensions/mcp-extensions.md | 2 +-
2 files changed, 2 insertions(+), 1 deletion(-)

Detailed changes

docs/src/extensions.md 🔗

@@ -8,3 +8,4 @@ Zed lets you add new functionality using user-defined extensions.
   - [Developing Themes](./extensions/themes.md)
   - [Developing Icon Themes](./extensions/icon-themes.md)
   - [Developing Slash Commands](./extensions/slash-commands.md)
+  - [Developing MCP Servers](./extensions/mcp-extensions.md)

docs/src/extensions/mcp-extensions.md 🔗

@@ -35,7 +35,7 @@ If you need to download the MCP server from an external source—like GitHub Rel
 
 ## Available Extensions
 
-Check out all the MCP servers that have already been exposed as Zed extensions [on Zed's site](https://zed.dev/extensions?filter=context-servers).
+Check out all the MCP servers that have already been exposed as extensions [on Zed's site](https://zed.dev/extensions?filter=context-servers).
 
 We recommend taking a look at their repositories as a way to understand how they are generally created and structured.