Extend documentation and toy tutorial with scheduling within the context of a building with PV

[Flix6x]

Follow-up from PR #451.

The tutorial for scheduling a toy battery can be followed up with an example of scheduling within the context of a building with local production from PV panels. If an optimization context isn't specified, the scheduler would exceed the available headroom as follows:

By setting the optimization context to take into account the building's local production (and self consumption, if any), the resulting schedule should no longer exceed the available headroom.

[Flix6x]

Continuing a discussion from https://github.com/FlexMeasures/flexmeasures/pull/451#discussion_r952911578, the EMS should be fully described in the optimization context, otherwise you might get weird outcomes if you only optimise a sub-part of the system.