Solve documentation

[ocots]

We could add examples with the most common options. Or a list: print_level, tolerances...

[ocots]

@PierreMartinon Feel free to update the tutorial or to list here the most common options you think we can present / list.

[ocots]

Here or elsewhere, we should add a word about:

• selecting the linear solver
• use of HSL.jl
• use of MKL.jl

[PierreMartinon]

Most common options are now described in the CTDirect API doc and the solve-tutorial.
More technical options such as linear solver choice / HSL / MKL are not presented yet.