Add documentation skeleton

Add documentation skeleton with:

• README.md
• CONTRIBUTING.md
• .gitignore
• mkdocs.yaml set with basic information
• .pre-commit.yaml set with following hooks:
  • no-commit-to-branch to main
  • trailing-whitespace
  • end-of-file-fixer
  • fix-byte-order-marker
  • yamllint
  • markdownlint
  • gitlint
• Gitlab-CI configuration in .ci/gitlab-ci.yaml with:
  • pre-commit hooks (with gitlint v0.17.0)
  • pages
• LICENSE

gitlint hook must remain in v0.17.0. Later versions are broken at the moment.