docs: use top base job to enable multiple environments

- multi-instances deployment of the same application.

This feature can be enabled using the [parallel matrix jobs](https://docs.gitlab.com/ee/ci/yaml/#parallelmatrix)

pattern at the `.os-env-base` job level (this is the top parent job of all deployment jobs).

pattern at the `.os-base` job level (the template's base job).

Environments namespacing is ensured by the `OS_ENVIRONMENT_NAMESPACE` variable (must start with a slash `/`).

Here is the example of the `.gitlab-ci.yml` file for a project deploying both a frontend and a backend applications:

```yaml

.os-env-base:

.os-base:

  parallel:

    matrix:

      - OS_ENVIRONMENT_NAMESPACE: "/front"