Document documentation contribution (#433)

docs/documentation.md

+# Documentation contribution
+
+<center>
+![MkDocs Material icon](../images/mkdocs-material-favicon.png)
+</center>
+
+The project documentation website is based on [MkDocs framework](https://www.mkdocs.org/) and on following theme and plugins:
+
+- [Mkdocs Material](https://squidfunk.github.io/mkdocs-material/): theme and framework
+- [Mike](https://github.com/jimporter/mike): to publish multiple versions
+- [MkDocs Coverage](https://github.com/pawamoy/mkdocs-coverage): to integrate coverage report iframe into MkDocs
+- [MkDocs GitLab plugin](https://pypi.org/project/mkdocs-gitlab-plugin/): to display links to GitLab references items: used in the changelog page
+
+Please refer to their respective documentation.
+
+## Set up
+
+Install the optional `docs` group:
+
+```
+poetry install --with docs
+```
+
+## MkDocs
+
+Run the website in development mode:
+
+```
+poetry run mkdocs deploy
+```
+
+Open [localhost:8000](http://localhost:8000/) in your web browser.
+
+The preview will be live-updated once you change the documentation.
+
+## Mike
+
+Running following command will serve what’s on `pages` branch:
+
+```
+poetry run mike serve
+```
+
+This preview won’t be live-updated with your latest changes as it would be with `mkdocs serve`. This is because Mike simply serves what’s built on `pages` branch.
+
+## Changing the content
+
+Into `docs` folder, you can find the Markdown source files which can be edited.
+`mkdocs.yml` file lists the website structure.
+
+## Deployment workflow
+
+Only on `main` branch, a `website` job will be triggered containing the documentation build for a specific version. It creates a commit which is added to `pages` branch with `poetry run mike deploy $version` command.
+Then the commit is pushed on the git repository remote from the job with an access token.
+The coverage report is integrated in the website build. The artefact is retrieved from the coverage job with `needs:` keyword.
+
+From `pages` branch, an other pipeline is triggered, publishing the documentation website to [silkaj.duniter.org](https://silkaj.duniter.org) for all versions via GitLab Pages.
+
+### Access token
+
+A `git push` is performed from the job to the remote repository via the authorisation provided via an access token set in CI/CD variables.
+The access token has to be renew every year, since this is the maximum lifespan we can set for an access token at its creation.
+The access token has to be created on a project basis with `repository_write` checked and with Developer role.