Expand toy tutorial with a sensor that reports available headroom

[Flix6x]

Follow-up from #534:

• Add a 'capacity' or 'headroom' sensor whose data is derived from an Asset attribute (capacity) in combination with sensor data (in this case that would only be the solar sensor). I feel that showing the headroom (like in the envisioned chart design of Extend documentation and toy tutorial with scheduling within the context of a building with PV #481) is what makes the example click.
• Add a chart showing the new schedule.

The headroom sensor relies on reporting functionality described in #509.

[nhoening]

In #760 I split the tutorial, so this should go into the second part.

[nhoening]

@victorgarcia98 wanted to use this as an opportunity to show the new reporting capabilities, so that will be an extra benefit of this tutorial (but also a little bit more work)

[nhoening]

This is a picture @victorgarcia98 made in the meantime, for reference.

[nhoening]

Hasn't this been solved in #825 ?

[Flix6x]

Almost. For this to close, toy example II should feature the relevant images, imo. Specifically, we should place two images at the end of toy example II (a before and after) showing the effect of taking into account solar (just showing the battery and the headroom sensors), with a forward reference to toy example IV, in which the headroom sensor is being set up.

[Flix6x]

We can also use this to reference to the page explaining how to customize what is visible in the chart on the asset page:

:ref: `view_asset-data`