docs: add github_repository_custom_properties resource documentation

Author: mkushakov

website/docs/r/repository_custom_properties.html.markdown

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+---
+layout: "github"
+page_title: "GitHub: github_repository_custom_properties"
+description: |-
+  Manages multiple custom property values for a GitHub repository
+---
+
+# github_repository_custom_properties
+
+This resource allows you to manage multiple custom property values for a GitHub repository in a single resource block. Property values are updated in-place when changed, without recreating the resource.
+
+~> **Note:** This resource manages **values** for custom properties that have already been defined at the organization level (e.g. using [`github_organization_custom_properties`](organization_custom_properties.html)). It cannot create new property definitions.
+
+~> **Note:** This resource requires the provider to be configured with an organization owner. Individual user accounts are not supported.
+
+## Example Usage
+
+```hcl
+resource "github_repository" "example" {
+  name = "example"
+}
+
+resource "github_repository_custom_properties" "example" {
+  repository_name = github_repository.example.name
+
+  property {
+    name  = "environment"
+    value = ["production"]
+  }
+
+  property {
+    name  = "team"
+    value = ["platform"]
+  }
+}
+```
+
+## Example Usage - Multi-Select Property
+
+```hcl
+resource "github_repository_custom_properties" "example" {
+  repository_name = "my-repo"
+
+  property {
+    name  = "languages"
+    value = ["go", "typescript", "python"]
+  }
+
+  property {
+    name  = "environment"
+    value = ["staging"]
+  }
+}
+```
+
+## Argument Reference
+
+The following arguments are supported:
+
+* `repository_name` - (Required) The name of the repository. Changing this will force the resource to be recreated.
+
+* `property` - (Required) One or more property blocks as defined below. At least one must be specified.
+
+### property
+
+* `name` - (Required) The name of the custom property. Must correspond to a property already defined at the organization level.
+
+* `value` - (Required) The value(s) for the custom property. This is always specified as a set of strings, even for non-multi-select properties. For `string`, `single_select`, `true_false`, and `url` property types, provide a single value. For `multi_select` properties, multiple values can be provided.
+
+## Attributes Reference
+
+In addition to all arguments above, the following attributes are exported:
+
+* `id` - A composite ID in the format `owner:repository_name`.
+
+## Import
+
+Repository custom properties can be imported using the `owner/repository_name` format. When imported, **all** custom property values currently set on the repository will be imported into state.
+
+```
+terraform import github_repository_custom_properties.example my-org/my-repo
+```
+
+## Differences from `github_repository_custom_property`
+
+This resource (`github_repository_custom_properties`, plural) manages **all** custom property values for a repository in a single resource block, with in-place updates when values change. This is useful when you want to manage multiple properties together as a unit.
+
+The singular [`github_repository_custom_property`](repository_custom_property.html) resource manages a **single** property value per resource instance. Use it when you need independent lifecycle management for each property.