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

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

Environments namespacing is ensured by the `AWS_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

.aws-env-base:

.aws-base:

  parallel:

    matrix:

      - AWS_ENVIRONMENT_NAMESPACE: "/front"

        # AWS deployment scripts are located in the ./front/ directory

        # Helm Chart is located in the ./front/ directory

        AWS_SCRIPTS_DIR: "front"

      - AWS_ENVIRONMENT_NAMESPACE: "/back"

        # AWS deployment scripts are located in the ./back/ directory

        # Helm Chart is located in the ./back/ directory

        AWS_SCRIPTS_DIR: "back"

# ⚠ on_stop must be unset when defining parallel:matrix environments