docs: update stale links to GitLab docs (anchors)

## Usage

This template can be used both as

a [CI/CD component](https://docs.gitlab.com/ee/ci/components/#use-a-component-in-a-cicd-configuration)

a [CI/CD component](https://docs.gitlab.com/ee/ci/components/#use-a-component)

or using the legacy [`include:project`](https://docs.gitlab.com/ee/ci/yaml/index.html#includeproject) syntax.

### Use as a CI/CD component

Here are some advices about your **secrets** (variables marked with a :lock:):

1. Manage them

   as [project or group CI/CD variables](https://docs.gitlab.com/ee/ci/variables/#create-a-custom-variable-in-the-ui):

   - [**masked**](https://docs.gitlab.com/ee/ci/variables/#mask-a-custom-variable) to prevent them from being

   as [project or group CI/CD variables](https://docs.gitlab.com/ee/ci/variables/#define-a-cicd-variable-in-the-ui):

   - [**masked**](https://docs.gitlab.com/ee/ci/variables/#mask-a-cicd-variable) to prevent them from being

     inadvertently

     displayed in your job logs,

   - [**protected**](https://docs.gitlab.com/ee/ci/variables/#protect-a-custom-variable) if you want to secure some

   - [**protected**](https://docs.gitlab.com/ee/ci/variables/#protect-a-cicd-variable) if you want to secure some

     secrets

     you don't want everyone in the project to have access to (for instance production secrets).

2. Manage them using the [Vault variant](#vault-variant)

3. In case a secret

   contains [characters that prevent it from being masked](https://docs.gitlab.com/ee/ci/variables/#masked-variable-requirements),

   contains [characters that prevent it from being masked](https://docs.gitlab.com/ee/ci/variables/#mask-a-cicd-variable),

   simply define its value as the [Base64](https://en.wikipedia.org/wiki/Base64) encoded value prefixed with `@b64@`:

   it will then be possible to mask it and the template will automatically decode it prior to using it.

4. Don't forget to escape special characters (ex: `$` -> `$$`).