GitLab

The CI builds, tags, create a release and a generic pages and finally
publishes to Gitlab pages automatically.
For development branches, it will only build.

  * Rely on requirement.txt
  * Use pelican's --listen mechanism instead of complex .sh file
  * Use python3 -m venv

publishconf now derives from pelicanconf, thus removing useless
duplications. At the same time, this actually restore two missing menus
in production: "about" and "contribute"

 * Move from JINJA_EXTENSIONS to JINJA_ENVIRONMENT
 * Remove non existing menu item /contact
 * Fix mastodon URL

There is no Windows version anymore, so this section was outdated.
Closes #23

Update introduction.md to use gender-neutral language and be proofread with the assistance of Grammarly

  - replace broken link
  - harmonize some styling elements
  - fix some typos

- write numbers in letter
- remove spaces before ponctuations (:, ?, !)
- remove double spaces
- use British English for consistency: licence
- remove French §
- remove duplicate article in the wiki section

Traduction PoW article + correct folder

See merge request websites/website_en!32