Consider adding docstrings to exported functions

[peanutfun]

The public API of the DataAssimilationBenchmarks.jl module is currently documented in the README.md. This clutters the document and does not adhere to a standardized documentation formatting.

Julia enables object documentation via docstrings, and advertises a standard to write such documentations. This has the advantage that the documentation is part of the actual source code, and that a formatted documentation can always be generated from the source code with Documenter.jl.

Please consider removing the API documentation from the README.md, and adding appropriate docstrings to the functions of the module that are part of the public API. I think this would work well together with my suggestion in #7, thus making the README.md a more "high-level" overview of the package.

openjournals/joss-reviews#4129

[cgrudz]

Many thanks, this has been completed adopted in the new documentation pages:
https://cgrudz.github.io/DataAssimilationBenchmarks.jl/dev/

[peanutfun]

The documentation really has improved considerably. Thanks a lot for all the effort! ✌️