Explain longoptions in the documentation

[Esteban82]

We have to update the documentation to include the long options (which are already available in the code).

I think we should do it before the release of GMT6.6 (#8653).

I am not sure how we should include them in the documentation. No doubt we should add a section in 3.General Features.

But for example, I don't know what should be done for the pages of each module. Whether to add a table with the conversion. Or write another page using only the long options (similar to what was done with the classic vs. modern syntax). What do you think?

I think we should discuss and agree on how we should update the documentation.