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 `.s3-env-base` job level (this is the top parent job of all deployment jobs).

pattern at the `.s3-base` job level (the template's base job).

Environments namespacing is ensured by the `S3_ENVIRONMENT_NAMESPACE` variable (must start with a slash `/`).

Here is the example of the `.gitlab-ci.yml` file for a project deploying 2 static websites (_doc_ and _dev-guide_):

```yaml

.s3-env-base:

.s3-base:

  parallel:

    matrix:

      - S3_ENVIRONMENT_NAMESPACE: /doc