ci: introduce `yamllint` checks

[tiborsimko]

It would be useful to enrich our CI check suite in order to validate YAML files.

We are using YAML in Helm charts, in REANA demo examples, but also in all the repositories with respect to commitlint.

In our deployment repository (using GitLab), we do:

(1) In GitLab CI:

lint-yamllint:
  image: registry.gitlab.com/pipeline-components/yamllint:latest
  script:
    - ./run-tests.sh --check-yamllint

(2) In run-tests.sh:

...

check_yamllint () {
    yamllint .
}

check_all () {
    check_black
    check_flake8
    check_pydocstyle
    check_shellcheck
    check_yamllint
}

for arg in "$@"
do
    case $arg in
        --check-black) check_black;;
        --check-flake8) check_flake8;;
        --check-pydocstyle) check_pydocstyle;;
        --check-shellcheck) check_shellcheck;;
        --check-yamllint) check_yamllint;;
        *) echo "invalid argument $arg" && exit 2;;
    esac
done

(3) We have the following .yamllint.yaml configuration file so that starting "---" is optional:

extends: default

rules:
  document-start: disable
  line-length: disable

The goal of this issue is to introduce yamllint in a similar way to all our GitHub repositories. This will consist of:

• enriching run-tests.sh so that the checking would work locally;
• finding out an appropriate GitHub action and enriching .github/workflows/ci.yml so that the checking would be always executed during pull request time.

[tiborsimko]

... and, when we are at enriching CI for all repositories, we could also introduce in the same PR set:

• markdownlint to check our *.md files that we also use everywhere in all the repositories;
• .editorconfig to introduce common editing style (tabs vs spaces, indentation length, etc).

I'll try to add some examples for the Markdown linting configuration and for the editor configuration later on.