Add a tutorial and update docstrings

[amontoison]

• Tutorial
• Update docstrings

[matbesancon]

For the tutorial, the one in the PDF https://github.com/ERGO-Code/basiclu/tree/master/doc could be a starter

[amontoison]

Thanks Mathieu, I already used it to answer a question on Discourse.
I want to add a tutorial here when I will find time but I only use BASICLU as a preconditioner in Krylov.jl for now (JuliaSmoothOptimizers/Krylov.jl#692).