Add technical documentation for allocation

[Jingru923]

Fixes #1540

[SouthEndMusic]

There are some 'words' in here that look like variables, like subnetwork_ids. I think for the documentation it is nicer if these variables are described with normal words

[Jingru923]

I was thinking about using variable-like name so it's easier for developer to look it up.

[gijsber]

I added some comments on a few places as the description was to vague to properly understand what was meant

[SouthEndMusic]

I added a last few notes, but overall its very nice now (: One last thing you can also add somewhere is that since a recent PR of mine, flow is first allocated to user demand nodes from the directly connected basin without optimization.