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/elixir.md

@@ -0,0 +1,43 @@
+# Elixir
+
+- Tree Sitter: [tree-sitter-elixir](https://github.com/elixir-lang/tree-sitter-elixir)
+- Language Server: [elixir-ls](https://github.com/elixir-lsp/elixir-ls)
+
+### Setting up `elixir-ls`
+
+1. Install `elixir`:
+
+```bash
+brew install elixir
+```
+
+2. Install `elixir-ls`:
+
+```bash
+brew install elixir-ls
+```
+
+3. Restart Zed
+
+{% hint style="warning" %}
+If `elixir-ls` is not running in an elixir project, check the error log via the command palette action `zed: open log`.  If you find an error message mentioning: `invalid LSP message header "Shall I install Hex? (if running non-interactively, use \"mix local.hex --force\") [Yn]`, you might need to install [`Hex`](https://hex.pm). You run `elixir-ls` from the command line and accept the prompt to install `Hex`.
+{% endhint %}
+
+### Formatting with Mix
+
+If you prefer to format your code with [Mix](https://hexdocs.pm/mix/Mix.html), use the following snippet in your `settings.json` file to configure it as an external formatter.  Formatting will occur on file save.
+
+```json
+{
+  "language_overrides": {
+    "Elixir": {
+      "format_on_save": {
+        "external": {
+          "command": "mix",
+          "arguments": ["format", "--stdin-filename", "{buffer_path}", "-"]
+        }
+      }
+    }
+  }
+}
+```