Move from Material for Mkdocs to Zensical

Migrated issue

Reported by: fjgarlin

Related to !448 (closed)

Problem/Motivation

Per this blog post, Material for MkDocs is now in maintenance mode only. There is a new tool called Zensical, by the same authors, which is supposed to be compatible.

Proposed resolution

Let's try the new tool and use it to build the sites. We'll need to update the documentation. According to the posts, the current mkdocs.yml and docs folders set up might just work.

We should offer the possibility of still running the legacy Material theme, so let's also create and document a variable that would force using the old theme, but default to the newest.

Example for GitLab pages in https://zensical.org/docs/publish-your-site/#gitlab-pages

pages:
  stage: deploy
  image: python:latest
  script:
    - pip install zensical
    - zensical build --clean 
  pages:
    publish: public
  rules:
    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'