Set up new website structure with MkDocs (#433)

Adapt current doc, Introduce MkDocs and plugins

• Adapt previous documentation structure to Mkdocs
• Introduce, configure, tweak Mkdocs and plugins/theme:
  • Material theme/framework, Mike (versions), coverage iframe integration, GitLab links

Set-up deployment workflow

• On main branch, in website job, build docs for a specific version, commit on pages branch, push commit on the remote from the job with access token
  • The coverage report is integrated in the website build. The artefact is retrieved from coverage job with needs: keyword
• From pages branch, an other pipeline is triggered, publishing the documentation website for all versions via GitLab Pages

Documentation content improvements

• Partially rework, and proofread current documentation
• Document documentation contribution

Close #433 (closed), #166 (closed)