update user guide

[KohlerHECTOR]

Description

PR for issues #325 #353 I remove some place holders in the user guide that we would probably never do and I did a user guide page for plotting and statitstics.

Checklist

• [ ] My code follows the style guideline To check : black --check examples rlberry *py flake8 --select F401,F405,D410,D411,D412 --exclude=rlberry/check_packages.py --per-file-ignores="init.py:F401",
• [ ] I have commented my code, particularly in hard-to-understand areas,
• [ ] I have made corresponding changes to the documentation,
• [ ] I have added tests that prove my fix is effective or that my feature works,
• [ ] New and existing unit tests pass locally with my changes,
• [ ] If updated the changelog if necessary,
• [ ] I have set the label "ready for review" and the checks are all green.