Add Partner Central Permission Sets mixin (#59)

* Add Partner Central Permission Sets mixin

This adds a new mixin file with 8 AWS Partner Central permission sets:
- PartnerCentralFullAccess
- PartnerCentralAccountMgmt
- PartnerCentralOpportunityMgmt
- PartnerCentralSandboxAccess
- PartnerCentralResourceSnapshot
- PartnerCentralChannelMgmt
- PartnerCentralHandshakeMgmt
- PartnerCentralMarketingMgmt

These permission sets enable AWS Partner Central integration and APN program management.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

* Add mixins documentation

Adds comprehensive documentation for using mixins with the aws-identity-center component:
- How to vendor mixins via component.yaml
- How to vendor mixins via vendor.yaml
- How to activate permission sets using additional-permission-sets_override.tf
- Pattern for creating custom mixins
- Detailed examples for all approaches

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

* docs: Add mixins usage documentation to README

- Add comprehensive "Using Mixins" section to README.yaml
- Document how to vendor mixins via component.yaml and vendor.yaml
- Explain how to activate permission sets using additional-permission-sets_override.tf
- Provide examples for creating custom mixins
- Regenerate README.md using atmos docs generate readme

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Benbentwo

README.md

@@ -145,6 +145,126 @@ usage: |-
   7. If you want the permission set to be able to use Terraform, enable access to the Terraform state read/write (default)
     role in `tfstate-backend`.
 
+  ### Using Mixins
+
+  Mixins provide a way to extend the component with additional permission sets without modifying the core component code.
+  This makes it easier to keep your components up-to-date with upstream changes while maintaining custom functionality.
+
+  #### Available Mixins
+
+  This component provides several mixins in the [`mixins/`](./mixins) directory:
+
+  - **`policy-PartnerCentral.tf`** - AWS Partner Central permission sets for AWS Partner Network (APN) integration
+
+  See the [mixins/README.md](./mixins/README.md) for a complete list of available mixins and detailed documentation.
+
+  #### Vendoring Mixins
+
+  **Option 1: Via component.yaml (Recommended)**
+
+  Add the mixin to your component's `component.yaml` file:
+
+  ```yaml
+  # components/terraform/aws-sso/component.yaml
+  apiVersion: atmos/v1
+  kind: ComponentVendorConfig
+  spec:
+    source:
+      uri: github.com/cloudposse-terraform-components/aws-identity-center.git//src?ref={{ .Version }}
+      version: 1.0.0
+      included_paths:
+        - "**/**"
+      excluded_paths: []
+
+    # Mixins are pulled and merged into your component directory
+    mixins:
+      - uri: github.com/cloudposse-terraform-components/aws-identity-center.git//mixins/policy-PartnerCentral.tf?ref={{ .Version }}
+        version: 1.0.0
+        filename: policy-PartnerCentral.tf
+  ```
+
+  **Option 2: Via vendor.yaml**
+
+  Use a centralized `vendor.yaml` file:
+
+  ```yaml
+  # vendor.yaml
+  apiVersion: atmos/v1
+  kind: AtmosVendorConfig
+  spec:
+    sources:
+      - component: "terraform/aws-sso"
+        source: "github.com/cloudposse-terraform-components/aws-identity-center.git//src?ref={{ .Version }}"
+        version: "1.0.0"
+        targets:
+          - "components/terraform/aws-sso"
+        mixins:
+          - source: "github.com/cloudposse-terraform-components/aws-identity-center.git//mixins/policy-PartnerCentral.tf?ref={{ .Version }}"
+            version: "1.0.0"
+            filename: "policy-PartnerCentral.tf"
+  ```
+
+  Then run:
+  ```bash
+  atmos vendor pull -c aws-sso
+  ```
+
+  #### Activating Vendored Permission Sets
+
+  After vendoring a mixin, include the permission sets in your component by updating `additional-permission-sets_override.tf`:
+
+  ```hcl
+  # components/terraform/aws-sso/additional-permission-sets_override.tf
+  locals {
+    # Add custom permission sets.
+    # Mixins define local variables (e.g., local.partner_central_permission_sets)
+    # that you concatenate into this list.
+    overridable_additional_permission_sets = concat(
+      local.partner_central_permission_sets,  # From policy-PartnerCentral.tf mixin
+      # Add other permission set locals here as needed
+      # local.custom_permission_sets,
+    )
+  }
+  ```
+
+  Each mixin defines a local variable containing its permission sets. For example, `policy-PartnerCentral.tf` defines
+  `local.partner_central_permission_sets` with 8 permission sets for AWS Partner Central.
+
+  #### Creating Custom Mixins
+
+  You can create your own mixin files following this pattern:
+
+  ```hcl
+  # components/terraform/aws-sso/policy-CustomRole.tf
+  locals {
+    custom_permission_sets = [
+      {
+        name                                = "MyCustomRole"
+        description                         = "Description of the role"
+        relay_state                         = ""
+        session_duration                    = ""
+        tags                                = {}
+        inline_policy                       = ""
+        policy_attachments                  = ["arn:${local.aws_partition}:iam::aws:policy/CustomPolicy"]
+        customer_managed_policy_attachments = []
+      },
+    ]
+  }
+  ```
+
+  Then reference it in `additional-permission-sets_override.tf`:
+
+  ```hcl
+  locals {
+    overridable_additional_permission_sets = concat(
+      local.custom_permission_sets,
+      local.partner_central_permission_sets,
+    )
+  }
+  ```
+
+  For more details, see [mixins/README.md](./mixins/README.md).
+
   #### Example
 
   The example snippet below shows how to use this module with various combinations (plain YAML, YAML Anchors and a