Create a docs page about 'space'

[glatterf42]

@behnam-zakeri thanks for filling in on this model feature that I wasn't really aware of!

As an aside, I think this points to the need for a 'space' docs page parallel to https://docs.messageix.org/en/latest/time.html, to explain such topics.

Originally posted by @khaeru in #845 (reply in thread)

This comment was a reaction to a question about how to apply bound_emission to all nodes under a parent node. It would be useful to create a dedicated docs page for this, collecting the best way to handle regions, spatial hierarchy, etc, which this issue tracks.

[glatterf42]

As discovered through #875, this new docs page should ideally also explain which features automatically integrate with the spatial hierarchy. I.e., prior to #845, neither @khaeru nor me knew that emissions could be bound on parent nodes if the spatial hierarchy is established correctly. In #875, we were again not sure if a commodity share constraint could be established for the parent nodes, and discovered that this is supported. It would be helpful to have a clearer overview of what is possible with this feature.