Add documentation

[tmigot]

We could add a tutorial that reflects the recommended use for GenericExecutionStats and logger.
For instance, a gradient descent method with fixed stepsize, that pre-allocate the stats beforehand, like a simplified version of what is done in JSOSolvers.jl.

[dpo]

Or simply the "dummy solver" used in the tests?

[tmigot]

better idea :). So, it might be also worth upgrading dummy_solver with

• pre-allocated solver structure
• verbose > 0 && @info log...
• and finally copy-paste it in the docs

[tmigot]

It can also be worth adding a link to this tutorial https://jso-docs.github.io/creating-a-jso-compliant-solver/ that @abelsiqueira made.