Update KeePassX documentation

Several steps changed:

• creating, managing and saving a database is done via the menu entry Database (previous menu File was removed)
• creation of a database is a bit different
• there is no Help ▸ KeePassX Handbook… anymore, so maybe we should point to the KeePassX website
• saving a database is a bit different (other menu name, text box name is File name and there is no Persistent entry in the left pane, but it’s in the list above)
• a persistent saved database with the name keepassx won’t be used as default after rebooting. Apparently it’s needed to open the database manually after starting KeePassX

Feature Branch: spriver:doc/11917-keepassx-2

Parent Task: #11916 (closed)

Subtasks

• #10956 (closed)

Related issues

• Blocked by #12368 (closed)

Original created by @spriver on 11917 (Redmine)