Added and modified readme files for autorest.az to use for codegen (Azure#14856)

* added and modified readme files for autorest.az

* removed autogen command groups

* addressed PR comments

Author: Alan Feng

specification/apimanagement/resource-manager/readme.az.md

@@ -0,0 +1,19 @@
+## AZ
+
+These settings apply only when `--az` is specified on the command line.
+
+``` yaml $(az) && $(target-mode) == 'core'
+tag: package-preview-2021-01
+az:
+    extensions: apim
+    namespace: azure.mgmt.apimanagement
+    package-name: azure-mgmt-apimanagement
+az-output-folder: $(azure-cli-folder)/src/azure-cli/azure/cli/command_modules/apim
+python-sdk-output-folder: $(az-output-folder)/vendored_sdks/apimanagement
+cli:
+    cli-directive:
+        - where:
+            group: "*"
+            op: "*"
+          hidden: true
+```

specification/apimanagement/resource-manager/readme.cli.md

@@ -0,0 +1,4 @@
+``` yaml
+# add any configuration here for all CLI languages
+# refer to the faq.md for more details
+```

specification/apimanagement/resource-manager/readme.md

@@ -516,7 +516,9 @@ See configuration in [readme.go.md](./readme.go.md)
 
 See configuration in [readme.java.md](./readme.java.md)
 
+## AZ
 
+See configuration in [readme.az.md](./readme.az.md)
 
 ## Suppression
 
@@ -606,4 +608,4 @@ directive:
   - suppress: R4009
     from: apimnotifications.json
     reason: Warning raised to error while PR was being reviewed. SystemData will implement in next preview version.  
-```
+```