Add rendering of rule markdown for terminal output (#2747)

Add rendering of rule markdown for terminal output
    
This is achieved by making use of the `mdcat` crate.
    
See the following links for details:
    
- https://crates.io/crates/mdcat
- https://github.com/swsnr/mdcat
- https://docs.rs/mdcat/latest/mdcat/

crates/ruff_cli/src/args.rs

@@ -41,7 +41,7 @@ pub enum Command {
         rule: Rule,
 
         /// Output format
-        #[arg(long, value_enum, default_value = "text")]
+        #[arg(long, value_enum, default_value = "markdown")]
         format: HelpFormat,
     },
     /// List all supported upstream linters
@@ -284,6 +284,7 @@ pub struct CheckArgs {
 pub enum HelpFormat {
     Text,
     Json,
+    Markdown,
 }
 
 #[allow(clippy::module_name_repetitions)]