docs: Add initial language settings documentation (#6957)

This pull request implements the following documentation changes:

- [x] Copy existing language settings docs from old docs repo
- [x] Add new pages for Zig, Haskell, Gleam, Deno and PureScript
- [x] Add `rust-analyzer` target directory section to Rust language page

Release Notes:

- Added initial language settings documentation
([#4264](https://github.com/zed-industries/zed/issues/4264)).

docs/src/languages/javascript.md

@@ -0,0 +1,25 @@
+# JavaScript
+
+- Tree Sitter: [tree-sitter-javascript](https://github.com/tree-sitter/tree-sitter-javascript)
+- Language Server: [typescript-language-server](https://github.com/typescript-language-server/typescript-language-server)
+
+### Code formatting
+
+Formatting on save is enabled by default for JavaScript, using TypeScript's built-in code formatting. But many JavaScript projects use other command-line code-formatting tools, such as [Prettier](https://prettier.io/). You can use one of these tools by specifying an _external_ code formatter for JavaScript in your settings. See the [configuration](../configuration/configuring-zed.md) documentation for more information.
+
+For example, if you have Prettier installed and on your `PATH`, you can use it to format JavaScript files by adding the following to your `settings.json`:
+
+```json
+{
+  "language_overrides": {
+    "JavaScript": {
+      "format_on_save": {
+        "external": {
+          "command": "prettier",
+          "arguments": ["--stdin-filepath", "{buffer_path}"]
+        }
+      }
+    }
+  }
+}
+```