doc(workflow): extend (skip ci) feature

Those variables are used internally thoughout all _to be continuous_ templates.

## Extended `[skip ci]` feature

GitLab skips triggering the CI/CD pipeline when `[skip ci]` or `[ci skip]` is present in the Git commit message.

This feature can be handy, but in some situations you would like to skip the CI/CD pipeline only under certain

circumstances. Example: when creating a release with a Git commit containing only _bumpversion_ changes (setting 

the new released version in configuration and/or documentation files). This commit will also be pushed as a tag. 

You might want to prevent the commit from being processed twice (one from the origin branch **and** one from the tag pipeline).

For this, all the recent versions of _to be continuous_ templates implement an extended `[skip ci]` feature.

It is now possible to skip **selectively** the CI/CD pipeline if your Git commit message contains a part of the following format:

```

[ci skip on <comma separated words>]

or:

[skip ci on <comma separated words>]

```

Supported words are:

| Words   | Description                |

| ------- | -------------------------- |

| `tag`   | skipped on tag pipelines |

| `mr`    | skipped on Merge Request pipelines |

| `mr`    | skipped on branch pipelines |

| `default` | skipped on the default project branch |

| `prod`  | skipped on the [production branch](#production-and-integration-branches) |

| `integ` | skipped on the [integration branch](#production-and-integration-branches) |

| `dev`   | skipped on any development branch (other than production or integration) |

## Merge Request workflow

One thing that has to be chosen with GitLab CI/CD is the [Merge Request workflow strategy](https://docs.gitlab.com/ee/ci/yaml/workflow.html#switch-between-branch-pipelines-and-merge-request-pipelines).

    # prevent branch pipeline when an MR is open (prefer MR pipeline)

    - if: '$CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS'

      when: never

    # the 7 next rules implement the extended [skip ci] feature

    - if: '$CI_COMMIT_MESSAGE =~ "/\[(ci skip|skip ci) on ([^],]*,)*tag(,[^],]*)*\]/" && $CI_COMMIT_TAG'

      when: never

    - if: '$CI_COMMIT_MESSAGE =~ "/\[(ci skip|skip ci) on ([^],]*,)*branch(,[^],]*)*\]/" && $CI_COMMIT_BRANCH'

      when: never

    - if: '$CI_COMMIT_MESSAGE =~ "/\[(ci skip|skip ci) on ([^],]*,)*mr(,[^],]*)*\]/" && $CI_MERGE_REQUEST_ID'

      when: never

    - if: '$CI_COMMIT_MESSAGE =~ "/\[(ci skip|skip ci) on ([^],]*,)*default(,[^],]*)*\]/" && $CI_COMMIT_REF_NAME =~ $CI_DEFAULT_BRANCH'

      when: never

    - if: '$CI_COMMIT_MESSAGE =~ "/\[(ci skip|skip ci) on ([^],]*,)*prod(,[^],]*)*\]/" && $CI_COMMIT_REF_NAME =~ $PROD_REF'

      when: never

    - if: '$CI_COMMIT_MESSAGE =~ "/\[(ci skip|skip ci) on ([^],]*,)*integ(,[^],]*)*\]/" && $CI_COMMIT_REF_NAME =~ $INTEG_REF'

      when: never

    - if: '$CI_COMMIT_MESSAGE =~ "/\[(ci skip|skip ci) on ([^],]*,)*dev(,[^],]*)*\]/" && $CI_COMMIT_REF_NAME !~ $PROD_REF && $CI_COMMIT_REF_NAME !~ $INTEG_REF'

      when: never

    # default: execute

    - when: always

```

    # prevent Merge Request pipeline

    - if: $CI_MERGE_REQUEST_ID

      when: never

    # /!\ you MAY keep the 7 rules implementing the extended [skip ci] feature

    # >> here <<

    - when: always

```