Addition of documentation for the household heat technology split initialization

[kaskranenburgQ]

This PR adds documentation for the initialization of the sliders in the section "Space heating & hot water" in the household section.

Discussion points:

• Placement of the file underneath Dataset manager.
  I don't know if this is the correct placement for this documentation.

• Inclusion of housing types in the Cyprus example
  The reason that the air heat pumps have a great share in comparison to electric boilers and biomass boilers is that they are not only used in the newest buildings, but in the newest buildings with the smallest heat demand (apartments, terraced houses, semi-detached houses). I feel this gives more detail, but might deviate the reader from the point.

• Overall styling.

Closes #210

[mabijkerk]

I made a first few quick changes:

• Fever determines the dataset initialization, I therefore would not say that it belongs to the Dataset Manager. I put it in its own category.
• Fever sets the heat demand and supply for the entire built environment. This means buildings / households and space heating / hot water. The basic functionality is the same, so we should be able to explain the basic principles and then highlight some details, or we give the example for space heating in households and then explain how the others differ.
• I recall @KoenvanB writing docs for all users on main/heat-built-environment, this might help us to determine what additional information is required for contributors.
• On one hand I like the example of Cyprus, but at the same time, I wonder whether we should make it more generic and more 'technical'. Many of the examples on the Contributors page are more generic, which makes them more robust (the label for Air heat pumps might change over the years). See the example for always on battery parks below:

- merit_order.group = dynamic: wind_inland
- merit_order.level = hv
- merit_order.relations.curtailment = park_curtailment
- merit_order.relations.output = park_output
- merit_order.relations.storage = park_storage
- merit_order.subtype = always_on_battery_park
- merit_order.type = producer

[kaskranenburgQ]

Thanks for the feedback @mabijkerk!

• Fever determines the dataset initialization, I therefore would not say that it belongs to the Dataset Manager. I put it in its own category.

I agree with this.

• Fever sets the heat demand and supply for the entire built environment. This means buildings / households and space heating / hot water. The basic functionality is the same, so we should be able to explain the basic principles and then highlight some details, or we give the example for space heating in households and then explain how the others differ.
• I recall @KoenvanB writing docs for all users on main/heat-built-environment, this might help us to determine what additional information is required for contributors.
• On one hand I like the example of Cyprus, but at the same time, I wonder whether we should make it more generic and more 'technical'. Many of the examples on the Contributors page are more generic, which makes them more robust (the label for Air heat pumps might change over the years).

After reading the documentation that @KoenvanB wrote, I see that the only thing that is not correct is the following passage:

The merit order then works by assigning the first technology in the producer order to the first consumer in the consumer order (i.e. new apartments for households), then to the next consumer, and so on. This process continues until the specified share of residences for the first technology has been reached. The ETM then continues this process with the second technology in the producer order, until ultimately all housing/building stock is assigned a technology based on the specified technology shares.

This is instead done based on the final demand for each technology and household type, the specified share of residences follows from this proces.

I see the following options:

1. We change this passage with a generic example (Basically the same as I wrote for Cyprus, but with generic rounded numbers (10,000 households and 1.0 PJ of energy), and labeling of technologies with 'Technology 1', 'Technology 2', etc.
2. We refer to the contributors page in this piece of text, and write the generic example in the contributors page.

[kndehaan]

First draft is looking good. I did some minor improvements.

The example of Cyprus works well, but I agree that we could make it more generic. Making it more technical would be useful as well, especially highlighting what the technology splits in the front-end in the default settings represent, for instance households_heater_heatpump_air_water_electricity_share:

- start_value_gql = present:V(households_space_heater_heatpump_air_water_electricity,"fever.share_in_group") * 100.0

And more technicalities / backend stuff in how the allocation process of technologies to residence type and construction period work..?

[kndehaan]

Looking very good! Only some minor comments.