Add tutorial for building new model

The [current documentation](https://energysystemsmodellinglab.github.io/MUSE2/user_guide.html) is a little sparse and it would be difficult for someone who'd never encountered MUSE1 to figure out how to build their own model.

MUSE1 has [great documentation](https://muse-os.readthedocs.io/en/latest/), including very thorough [tutorials](https://muse-os.readthedocs.io/en/latest/user-guide/index.html). It would be great if we could have something similar for MUSE2.

One approach would be to literally port one or more of the tutorials from MUSE1 to MUSE2. Note that this may not be trivial given the differences between MUSE1 and MUSE2 (and may not be possible in some cases, if the relevant features are not available in MUSE2), but it may be a good starting point.

If anyone takes this, they should ask @tsmbland about it first.