docs: add documentation for the fmt command.

site/content/docs/intro/cli.md

@@ -34,6 +34,7 @@ Commands:
   scan        Scan a file or directory
   compile     Compile rules to binary form
   dump        Show the data produced by YARA modules for a file
+  fmt         Format YARA source files
   completion  Output shell completion code for the specified shell
   help        Print this message or the help of the given subcommand(s)
 
@@ -299,7 +300,7 @@ This option can be used multiple times for specifying more than one module.
 For example:
 
 ```
-yr dump --module=pe --module=dotnet [FILE]
+yr dump --module=pe --module=dotnet <FILE>
 ```
 
 ### --no-colors
@@ -309,4 +310,21 @@ Turn off output colors.
 By default, both YAML and JSON outputs contains colors that improves their
 legibility, this option turns off colors. When the output of this command is
 redirected from stdout to a file, colors are turned off automatically, even
-if `--no-colors` is missing.
+if `--no-colors` is missing.
+
+## fmt
+
+Format YARA source files.
+
+This command is similar in spirit to other code formatting tools like `gofmt`
+and `rustfmt`.
+
+```
+yr fmt <FILE>...
+```
+
+### ---check, -c
+
+Run in "check" mode. Doesn't modify any file, but exits error code 0 if the
+files are formatted correctly and no change is necessary, or error code 1
+if otherwise.