* release.config.js: declare 3 variables for `stable`, `testing` and
  `dev` branches.
  By default, `testingBranch` and `devBranch` are undefined and users
  only need to declare them to enable prerelease for them.

When doing backport of evolutions from other project, the `branches`
parameter was reset to `master`.

* Dockerfile.semantic-release-gitlab: install `semantic-release-pypi`
  and it's dependencies.

* release.config.js: add a commented example of the configuration of
  `semantic-release-pypi`.

The `commitlint` and `semantic-release` configuration files couldn't
be used as-is in projects running `eslint` on all JS files.

By default, `semantic-release` build changelog even for prerelease
branches which looks like duplicated entries to users.

Use a workaround to avoid changelog generation for prerelease branches
by building the configuration in several steps instead of a direct
`module.exports` at the top.

There are no effect on this project since the `branches` configuration
attribute only contains `stable` but can be used for other project
with the only change to declare prerelease `branches`.

* release.config.js: rework the configuration file to conditionnally
  execute `@semantic-release/changelog` and add its assets to the git
  plugin.

Create new release when merge requests are merged into the
`$STABLE_BRANCH` branch.

* templates/Release/Semantic-release.yaml: run `semantic-release` with
  the latest EOLE container for `$STABLE_BRANCH` branch if one of its
  configuration is present.

* .gitlab-ci.yml: execute a new stage `release` for
  `semantic-release`.

* release.config.js: create `docs/CHANGELOG.md` and publish new
  releases on Gitlab.
  Mangle commits to have links for users, issues and commits id.

* release-rules.js: create semantic version update rules based on
  `docs/CONTRIBUTING.md`.