Merge scalafix doc into fix (#3420)

website/docs/commands/fix.md

@@ -10,10 +10,56 @@ You can pass it explicitly or set it globally by running:
     scala-cli config power true
 :::
 
-The `fix` command scans your project for `using` directives and extracts them into the `project.scala` file placed in the project root directory.
-This allows to fix warnings tied to having `using` directives present in multiple files.
-Additionally, `fix` will format the result file, thus allowing to quickly spot configuration options that may be duplicated.
+The `fix` command is used to check, lint, rewrite or otherwise rearrange code in a Scala CLI project.
 
-Files containing `using target` directives, e.g. `//> using target.scala 3.0.0` will not be changed by `fix`.
+Currently, the following sets of rules are supported:
+- built-in rules (enabled automatically and controlled with the `--enable-built-in` flag)
+- `scalafix`, running [Scalafix](https://scalacenter.github.io/scalafix/) under the hood (enabled automatically and controlled with `--enable-scalafix` flag).
 
-The command respects the original scope of each extracted directive and will transform them into their `test.` equivalent if needed.
+You can disable unnecessary rule sets when needed.
+For example, to disable built-in rules, you can run:
+```bash
+scala-cli fix . --power --enable-built-in=false
+```
+
+## Built-in rules
+
+Currently, the only built-in rule is extraction of `using` directives into the `project.scala` configuration file.
+This allows to fix warnings tied to having `using` directives present in multiple files and eliminate duplicate directives.
+Files containing (experimental) `using target` directives, e.g. `//> using target.scala 3.0.0` will not be changed by `fix`.
+The original scope (`main` or `test`) of each extracted directive is respected. `main` scope directives are transformed 
+them into their `test.*` equivalent when needed.
+
+## `scalafix` integration
+
+Scala CLI is capable of running [Scalafix](https://scalacenter.github.io/scalafix/) (a refactoring and linting tool for Scala) on your project.
+Before using this command you need to provide the configuration at `.scalafix.conf`.
+For example:
+``` text title=.scalafix.conf
+// .scalafix.conf
+rules = [
+  DisableSyntax
+]
+```
+
+Then you can run it:
+```bash
+scala-cli fix . --power
+```
+
+If you’re setting up a continuous integration (CI) server, Scala CLI also has you covered.
+You can run linter using the `--check` flag:
+```bash fail
+scala-cli fix --check . --power
+```
+
+Read more about Scalafix:
+- [Configuration](https://scalacenter.github.io/scalafix/docs/users/configuration.html)
+- [Rules](https://scalacenter.github.io/scalafix/docs/rules/overview.html)
+
+### Using external rules
+
+Adding an [external scalafix rule](https://scalacenter.github.io/scalafix/docs/rules/external-rules.html) to Scala CLI can be done with the [`scalafix.dep`](./compile.md#compile-only-dependencies) directive:
+```scala compile power
+//> using scalafix.dep com.github.xuwei-k::scalafix-rules:0.5.1
+```