ci(commitlint): enforce commit message format

We want to generate release automatically when the `$STABLE_BRANCH`
branch is pushed.

We will use `semantic-release` to do the jobs and we must make sure
the commits will be taken into account by the automatic process.

* docs/CONTRIBUTING.md: explain the commit message formatting.

* .gitlab-ci.yml: configure the `$STABLE_BRANCH` as `master` instead
  of default `stable`.
  Execute `commitlint` at `initial-checks` stage.

* commitlint.config.js: configure `commitlint` policy.