Hydrogen improvements documentation

[kndehaan]

Modelling of hydrogen supply, demand and storage has been improved. Summarised:

• A break down in must-run and dispatchable plants of the hydrogen supply technologies ATR, SMR and ammonia reforming
• Possibility to change the merit order of dispatchable hydrogen production and demand
• Modelling of hydrogen storage is improved by adding salt cavern and depleted gas field technologies and allowing the user to set installed volume and relative capacity.
• Changes in hydrogen transport assumptions and modelling

• New charts: "Installed storage volume per carrier" and "Flexible hydrogen supply and demand per hour"

  Changes applied in this repo specifically are:

• Changed/added documentation on hydrogen supply, storage technologies, merit order of dispatchable supply and demand
• Added new charts and slider screenshots

Goes with:

• https://github.com/quintel/etsource/pull/3021
• https://github.com/quintel/etmodel/pull/4223
• https://github.com/quintel/documentation/pull/175
• https://github.com/quintel/etdataset/pull/994
• https://github.com/quintel/atlas/pull/166

[kndehaan]

I've pushed some text additions and changes with regard to @mabijkerk's requested changes: https://github.com/quintel/documentation/pull/175/commits/1ff3652e019491e35ed3e7c9d7c18005c2a74d0d