Use CI_DEFAULT_BRANCH in CI script instead of master for pages deployment

Main change

• Use $CI_DEFAULT_BRANCH in CI script instead of master for pages deployment

  The current master hardcoded value prevents projects using main as default branch to correctly deploy GitLab Pages

Extra changes

• Add option to select jinja2 version to avoid issue with MkDocs/jinja2 version mismatch (https://github.com/readthedocs/readthedocs.org/issues/9064) Unnecessary since this issue was fixed in MkDocs 1.2.4
• Improve documentation about variables in README file