TBA to a different MR/issue

For ssot epic

For CI services examples

as requested by https://gitlab.com/gitlab-org/gitlab-ce/issues/30477#note_29060568 in #30477

For ssot epic

Even though there's no relation to the feature (it's actually about
overriding template values, best to replace this with
$KUBE_INGRESS_BASE_DOMAIN to avoid any potential confusion

Update documentation with example how to use CI variable types.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/46806.

Includes missing documentation for 'Play all manual' builds a single
stage.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/28741

added `before_script`
as it failed to build maven due to insufficient permissions

Update mysql.md: variables values should be quoted, otherwise you get an "variables config should be a hash of key value pairs" error

Related to gitlab-ee/!5566

Ruby 2.6 offers better performance and memory usage:
https://www.rubyguides.com/2018/11/ruby-2-6-new-features/

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/57323

Add `Variable Types` section to explain supported variable types and
update example and related screenshots.

Update test-and-deploy-python-application-to-heroku.md

Plus: add note for .com users