Add Go Proxy API docs

doc/api/packages/go_proxy.md

+---
+stage: Package
+group: Package
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Go Proxy API
+
+This is the API documentation for [Go Packages](../../user/packages/go_proxy/index.md).
+This API is behind a feature flag that is disabled by default. GitLab administrators with access to
+the GitLab Rails console can [enable](../../administration/feature_flags.md)
+this API for your GitLab instance.
+
+WARNING:
+This API is used by the [Go client](https://maven.apache.org/)
+and is generally not meant for manual consumption.
+
+For instructions on how to work with the Go Proxy, see the [Go Proxy package documentation](../../user/packages/go_proxy/index.md).
+
+NOTE:
+These endpoints do not adhere to the standard API authentication methods.
+See the [Go Proxy package documentation](../../user/packages/go_proxy/index.md)
+for details on which headers and token types are supported.
+
+## List
+
+> Introduced in GitLab 13.1.
+
+Get all tagged versions for a given Go module:
+
+```plaintext
+GET projects/:id/packages/go/:module_name/@v/list
+```
+
+| Attribute      | Type   | Required | Description |
+| -------------- | ------ | -------- | ----------- |
+| `id`           | string | yes      | The project ID or full path of a project. |
+| `module_name`  | string | yes      | The name of the Go module. |
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/projects/1/packages/go/my-go-module/@v/list"
+```
+
+Example output:
+
+```shell
+"v1.0.0\nv1.0.1\nv1.3.8\n2.0.0\n2.1.0\n3.0.0"
+```
+
+## Version metadata
+
+> Introduced in GitLab 13.1.
+
+Get all tagged versions for a given Go module:
+
+```plaintext
+GET projects/:id/packages/go/:module_name/@v/:module_version.info
+```
+
+| Attribute         | Type   | Required | Description |
+| ----------------- | ------ | -------- | ----------- |
+| `id`              | string | yes      | The project ID or full path of a project. |
+| `module_name`     | string | yes      | The name of the Go module. |
+| `module_version`  | string | yes      | The version of the Go module. |
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/projects/1/packages/go/my-go-module/@v/1.0.0.info"
+```
+
+Example output:
+
+```json
+{
+  "Version": "v1.0.0",
+  "Time": "1617822312 -0600"
+}
+```
+
+## Download module file
+
+> Introduced in GitLab 13.1.
+
+Fetch the `.mod` module file:
+
+```plaintext
+GET projects/:id/packages/go/:module_name/@v/:module_version.mod
+```
+
+| Attribute         | Type   | Required | Description |
+| ----------------- | ------ | -------- | ----------- |
+| `id`              | string | yes      | The project ID or full path of a project. |
+| `module_name`     | string | yes      | The name of the Go module. |
+| `module_version`  | string | yes      | The version of the Go module. |
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/projects/1/packages/go/my-go-module/@v/1.0.0.mod"
+```
+
+Write to a file:
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/projects/1/packages/go/my-go-module/@v/1.0.0.mod" >> foo.mod
+```
+
+This writes to `foo.mod` in the current directory.
+
+## Download module source
+
+> Introduced in GitLab 13.1.
+
+Fetch the `.zip` of the module source:
+
+```plaintext
+GET projects/:id/packages/go/:module_name/@v/:module_version.zip
+```
+
+| Attribute         | Type   | Required | Description |
+| ----------------- | ------ | -------- | ----------- |
+| `id`              | string | yes      | The project ID or full path of a project. |
+| `module_name`     | string | yes      | The name of the Go module. |
+| `module_version`  | string | yes      | The version of the Go module. |
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/projects/1/packages/go/my-go-module/@v/1.0.0.zip"
+```
+
+Write to a file:
+
+```shell
+curl --header "Private-Token: <personal_access_token>" "https://gitlab.example.com/api/v4/projects/1/packages/go/my-go-module/@v/1.0.0.zip" >> foo.zip
+```
+
+This writes to `foo.zip` in the current directory.

doc/user/packages/go_proxy/index.md

@@ -16,6 +16,9 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 With the Go proxy for GitLab, every project in GitLab can be fetched with the
 [Go proxy protocol](https://proxy.golang.org/).
 
+For documentation of the specific API endpoints that the Go Proxy uses, see the
+[Go Proxy API documentation](../../../api/packages/go_proxy.md).
+
 ## Enable the Go proxy
 
 The Go proxy for GitLab is under development, and isn't ready for production use