Rewrite slide text for Electricity storage

[mabijkerk]

Background
The slide text for Electricity storage in the Flexibility section is quite extensive:

Issue
The text is so extensive that it no longer fits within the screen view. This means that the following slides, which actually contain the sliders, are hidden from view.

Solution
Rewrite this slide to a much shorter version, referring mostly to the documentation. Check the documentation for completeness and clarity. On one item the docs already fall short: they don't have any information about forecasting for local households.