Update documentation of Flexibility section to align it with ETM updates

[mabijkerk]

I have rewritten part of the Flexibility documentation to align it with final changes made to the 'need for flexibility' narrative in the ETM. The structure of the updated documentation is as follows:

• Introduction to flexibility and defining the goal of the documentation [minor changes]
• Distinction between flexibility for volume and flexibility for capacity [minor changes]
• Distinction between inflexible and flexible supply and demand [expanded, major changes]
• Explanation of the purpose of the 'need for flexibility' narrative [slimmed down, major changes]
• Overview of flexible and inflexible technologies for the energy carriers [minor changes]

@marliekeverweij can you check what you think of this updated documentation? One of the key things that I have changed is removing the explanation of each chart in the 'need for flexibility' narrative, because these are already explained in the ETM and are therefore redundant. Instead I have simply outlined the purpose of the narrative, so please check whether you think it has added value.

[mabijkerk]

FYI - I have changed some of the titles, so the links in the ETM to these sections of the Documentation have to be updated as well when this PR is merged.

[marliekeverweij]

Very nice :)

[marliekeverweij]

I will adjust the links in the ETM