[RTD] Update and expand references

[smartie2076]

We should update the literature/reference section of the RTD.

New source:

Fosse, Veslemøy (2020). Techno-economic analysis of solar power potential in Small Island Developing States. University of Freiburg, Freiburg, Germany.  

The thesis used Offgridders to find the optimal energy systems solutions for Small Island Developing States. Simulations included energy systems using solar power, diesel generators and battery storage. The results were used to investigate if solar power is cost-competitive compared to conventional energy and a feasible option for providing electricity on the islands.