Set the -J optional

[Esteban82]

This PR updates the documentation to indicate that the -J option is optional.

[Esteban82]

See discussion in #8695

[Esteban82]

Is there someone that would like to revise this?

[Esteban82]

Should I merge this directly? or someone would like to revise it?

[joa-quim]

Don't remember when -J stop being mandatory to some modules, and not sure where that happens also. For example in psbasemap we have this line

n_errors += gmt_M_check_condition (GMT, !GMT->common.J.active, "Must specify a map projection with the -J option\n");

but still that is not activated when -J is not used.

Also, we cannot just move -J to optional and not explaining what happens in the default case. explain_-J.rst_ does not say a word about that and probably cannot because that file is included in modules where -J is mandatory (grdproject, etc...).

At the end, I would perhaps just leave things as they are. They have served us well for > 30 years