Add Helm API documentation

doc/api/packages/helm.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
+---
+
+# Helm API
+
+This is the API documentation for [Helm](../../user/packages/helm_repository/index.md).
+
+WARNING:
+This API is used by the Helm-related package clients such as [Helm](https://helm.sh/)
+and [`helm-push`](https://github.com/chartmuseum/helm-push/#readme),
+and is generally not meant for manual consumption. This API is under development and is not ready
+for production use due to limited functionality.
+
+For instructions on how to upload and install Helm packages from the GitLab
+Package Registry, see the [Helm registry documentation](../../user/packages/helm_repository/index.md).
+
+NOTE:
+These endpoints do not adhere to the standard API authentication methods.
+See the [Helm registry documentation](../../user/packages/helm_repository/index.md)
+for details on which headers and token types are supported.
+
+## Enable the Helm API
+
+The Helm API for GitLab is behind a feature flag that is disabled by default. GitLab
+administrators with access to the GitLab Rails console can enable this API for your instance.
+
+To enable it:
+
+```ruby
+Feature.enable(:helm_packages)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:helm_packages)
+```
+
+## Download a chart index
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/62757) in GitLab 14.1.
+
+Download a chart index:
+
+```plaintext
+GET projects/:id/packages/helm/:channel/index.yaml
+```
+
+| Attribute | Type   | Required | Description |
+| --------- | ------ | -------- | ----------- |
+| `id`      | string | yes      | The ID or full path of the project. |
+| `channel` | string | yes      | Helm repository channel. |
+
+```shell
+curl --user <username>:<personal_access_token> \
+     https://gitlab.example.com/api/v4/projects/1/packages/helm/stable/index.yaml
+```
+
+Write the output to a file:
+
+```shell
+curl --user <username>:<personal_access_token> \
+     https://gitlab.example.com/api/v4/projects/1/packages/helm/stable/index.yaml \
+     --remote-name
+```
+
+## Download a chart
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61014) in GitLab 14.0.
+
+Download a chart:
+
+```plaintext
+GET projects/:id/packages/helm/:channel/charts/:file_name.tgz
+```
+
+| Attribute   | Type   | Required | Description |
+| ----------- | ------ | -------- | ----------- |
+| `id`        | string | yes      | The ID or full path of the project. |
+| `channel`   | string | yes      | Helm repository channel. |
+| `file_name` | string | yes      | Chart file name. |
+
+```shell
+curl --user <username>:<personal_access_token> \
+     https://gitlab.example.com/api/v4/projects/1/packages/helm/stable/charts/mychart.tgz \
+     --remote-name
+```
+
+## Upload a chart
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64814) in GitLab 14.1.
+
+Upload a chart:
+
+```plaintext
+POST projects/:id/packages/helm/api/:channel/charts
+```
+
+| Attribute | Type   | Required | Description |
+| --------- | ------ | -------- | ----------- |
+| `id`      | string | yes      | The ID or full path of the project. |
+| `channel` | string | yes      | Helm repository channel. |
+| `chart`   | file   | yes      | Chart (as `multipart/form-data`). |
+
+```shell
+curl --request POST \
+     --form 'chart=@mychart.tgz' \
+     --user <username>:<personal_access_token> \
+     https://gitlab.example.com/api/v4/projects/1/packages/helm/api/stable/charts
+```

doc/user/packages/helm_repository/index.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
+---
+
+# Helm charts in the Package Registry **(FREE)**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/18997) in GitLab 14.1.
+
+WARNING:
+The Helm package registry for GitLab is under development and isn't ready for production use due to
+limited functionality.
+
+Publish Helm packages in your project's Package Registry. Then install the
+packages whenever you need to use them as a dependency.
+
+For documentation of the specific API endpoints that Helm package manager
+clients use, see the [Helm API documentation](../../../api/packages/helm.md).
+
+## Enable the Helm repository feature
+
+Helm repository support is still a work in progress. It's gated behind a feature flag that's
+**disabled by default**. [GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can opt to enable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:helm_packages)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:helm_packages)
+```
+
+## Build a Helm package
+
+Creating a Helm package is documented [in the Helm documentation](https://helm.sh/docs/intro/using_helm/#creating-your-own-charts).
+
+## Authenticate to the Helm repository
+
+To authenticate to the Helm repository, you need either:
+
+- A [personal access token](../../../api/index.md#personalproject-access-tokens).
+- A [deploy token](../../project/deploy_tokens/index.md).
+- A [CI/CD job token](../../../api/index.md#gitlab-cicd-job-token).
+
+## Publish a package
+
+Once built, a chart can be uploaded to the `stable` channel with `curl` or `helm-push`:
+
+- With `curl`:
+
+  ```shell
+  curl --request POST \
+       --form 'chart=@mychart.tgz' \
+       --user <username>:<personal_access_token> \
+       https://gitlab.example.com/api/v4/projects/1/packages/helm/api/stable/charts
+  ```
+
+- With the [`helm-push`](https://github.com/chartmuseum/helm-push/#readme) plugin:
+
+  ```shell
+  helm repo add --username <username> --password <personal_access_token> project-1 https://gitlab.example.com/api/v4/projects/1/packages/helm/stable
+  helm push mychart.tgz project-1
+  ```
+
+## Install a package
+
+To install the latest version of a chart, use the following command:
+
+```shell
+helm repo add project-1 https://gitlab.example.com/api/v4/projects/1/packages/helm/stable
+helm install my-release project-1/mychart
+```
+
+See [Using Helm](https://helm.sh/docs/intro/using_helm/) for more information.