fix(pages): handle mkdocs output folder to already be 'public'

### GitLab Pages variant

This variant adds a job to publish the MkDocs generated site to GitLab pages (on `master` branch only).

Basically it copies the content of the mkdocs generated site folder (`site` by default) to the `public` folder which is published by [GitLab pages](https://docs.gitlab.com/ee/user/project/pages/#how-it-works).

If you wish to use it, add the following to your `gitlab-ci.yml`:

stages:

  - publish

.mkdocs-scripts: &mkdocs-scripts |

  # BEGSCRIPT

  set -e

  function log_info() {

      echo -e "[\\e[1;94mINFO\\e[0m] $*"

  }

  function log_warn() {

      echo -e "[\\e[1;93mWARN\\e[0m] $*"

  }

  function log_error() {

      echo -e "[\\e[1;91mERROR\\e[0m] $*"

  }

  # ENDSCRIPT

pages:

  stage: publish

  before_script:

    - *mkdocs-scripts

  script:

    - mkdir -p public

    - cp -r ${MKD_WORKSPACE_DIR:-.}/${MKD_SITE_DIR:-site}/* public/

    - mkdocs_dist=${MKD_WORKSPACE_DIR:-.}/${MKD_SITE_DIR:-site}

    - target="public/"

    - |

      if [ "${mkdocs_dist}" -ef "${target}" ]

      then

        log_info "configured mkdocs site dir already is pages expected folder"

      else

        log_info "copying mkdocs generated site from \\e[33;1m${mkdocs_dist}\\e[0m to pages expected folder \\e[33;1m${target}\\e[0m"

        mkdir -p public

        cp -r ${mkdocs_dist}/* ${target}

      fi

  artifacts:

    paths:

      - public