Write ruff server setup guide for Helix (#11183)

## Summary

Closes #11027.

crates/ruff_server/docs/setup/HELIX.md

@@ -0,0 +1,61 @@
+## Helix Setup Guide for `ruff server`
+
+First, open the language configuration file for Helix. On Linux and macOS, this will be at `~/.config/helix/languages.toml`,
+and on Windows this will be at `%AppData%\helix\languages.toml`.
+
+Add the language server by adding:
+
+```toml
+[language-server.ruff]
+command = "ruff"
+args = ["server", "--preview"]
+```
+
+Then, you'll register the language server as the one to use with Python.
+If you don't already have a language server registered to use with Python, add this to `languages.toml`:
+
+```toml
+[[language]]
+name = "python"
+language-servers = ["ruff"]
+```
+
+Otherwise, if you already have `language-servers` defined, you can simply add `"ruff"` to the list. For example,
+if you already have `pylsp` as a language server, you can modify the language entry as follows:
+
+```toml
+[[language]]
+name = "python"
+language-servers = ["ruff", "pylsp"]
+```
+
+> \[!NOTE\]
+> Multiple language servers for a single language are only supported in Helix version [`23.10`](https://github.com/helix-editor/helix/blob/master/CHANGELOG.md#2310-2023-10-24) and later.
+
+Once you've set up the server, you should see diagnostics in your Python files. Code actions and other LSP features should also be available.
+
+![image](assets/SuccessfulHelixSetup.png "A screenshot showing an open Python file in Helix with highlighted diagnostics and a code action dropdown menu open")
+*This screenshot is using `select=["ALL]"` for demonstration purposes.*
+
+If you want to, as an example, turn on auto-formatting, add `auto-format = true`:
+
+```toml
+[[language]]
+name = "python"
+language-servers = ["ruff", "pylsp"]
+auto-format = true
+```
+
+See the [Helix documentation](https://docs.helix-editor.com/languages.html) for more settings you can use here.
+
+You can pass settings into `ruff server` using `[language-server.ruff.config.settings]`. For example:
+
+```toml
+[language-server.ruff.config.settings]
+line-length = 80
+[language-server.ruff.config.settings.lint]
+select = ["E4", "E7"]
+preview = false
+[language-server.ruff.config.settings.format]
+preview = true
+```