Improve documentation for back-up

[math-GH]

Let's improve the documentation https://freshrss.github.io/FreshRSS/en/admins/04_Updating.html:

Add information which folders are really important to back-up. I would say only ./data/ is crucial. The ./extensions/ folder is also important to keep the extensions. Optional custom themes in ./i/themes/, if added. All other folders should be source folders an covered by a default installation.

[zoredache]

Maybe also a reminder to backup your external database if you are using mysql/mariadb/etc.

[Alkarex]

For databases, I recommand our built-in export-to-sqlite command