zig: Extract to zed-extensions/zig repository (#26569)

This PR extracts the Zig extension to the
[zed-extensions/zig](https://github.com/zed-extensions/zig) repository.

Release Notes:

- N/A

docs/src/languages/zig.md

@@ -1,6 +1,6 @@
 # Zig
 
-Zig support is available through the [Zig extension](https://github.com/zed-industries/zed/tree/main/extensions/zig).
+Zig support is available through the [Zig extension](https://github.com/zed-extensions/zig).
 
 - Tree-sitter: [tree-sitter-zig](https://github.com/tree-sitter-grammars/tree-sitter-zig)
 - Language Server: [zls](https://github.com/zigtools/zls)