Add gitlab templates for some website jobs

Consider this as a suggestion, feedback is welcome!

This is a first draft to port the Jenkins website jobs for Hugo and Sphinx app docs to GitLab.

I am not sure if this how we like to do it, I am especially unsure about the ssh/deploy part. It requires to setup the ssh key as a variable for each repository, that needs to deploy a website.

Of course I couldn't test it with KDE infrastructure, but I tested it with GitLab.com and my private server.

---

Usage Examples

For sphinx:

include: https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/website-sphinx-app-docs.yml

variables:
    EPUB_NAME: KdenliveManual
    WEBSITE_DOMAIN: docs.kdenlive.org

build_german_docs:
    extends: .build_sphinx_app_docs
    variables:
      BUILD_LANGUAGE: de

For simple hugo:

include: https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/website-hugo.yml

variables:
    WEBSITE_DOMAIN: develop.kde.org

Hugo for the cases of where we have staging (probably possible to simplify that a bit more):

include: https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/website-hugo.yml

variables:
  WEBSITE_DOMAIN: kde.org

workflow:
  rules:
    - if: $CI_COMMIT_BRANCH == "work/staging" || $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "work/staging"
      variables:
        WEBSITE_DOMAIN: www-staging.kde.org
        HUGO_BASEURL: https://www-staging.kde.org/
        HUGO_ADDIONAL_ARGS: "--buildFuture --buildDrafts"
    # Prevent branch pipelines if an MR is open on the branch.
    - if: $CI_COMMIT_BRANCH && $CI_PIPELINE_SOURCE == "push" && $CI_OPEN_MERGE_REQUESTS
      when: never
    # Allow merge request pipelines.
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
    # Build tags and branches too
    - if: $CI_COMMIT_TAG
    - if: $CI_COMMIT_BRANCH