[DOCS] Document sources and details for all input data

[maartenbrinkerink]

Type of documentation issue.

Missing

Link to existing documentation

No response

Issue Description

Even though we make use of solely open data, the origin (and assumptions) of input data is not always easily retrievable (i.e. it requires going into individual files and/or scripts). Furthermore, it would be useful to go through the data sources to check which data sources can be converted from static outdated files to dynamic links that can be updated e.g. yearly. For example, some data we use in the 'demand_projections' script only runs until 2020 and requires updating as mentioned in #189.

Suggested Update

-To have a section in the documentation with at minimum the used data sources, for which variables it is relevant, which data years the data is based on, for which geographical scope it is applicable and any relevant assumptions that have been made.
-To convert hardcoded/static links to data into dynamic links that can be updated automatically.

Additional Info

No response