DOC: Combine the modern and classic ReST files into a single file about gmt

[Esteban82]

I like it. I think we should implemen it.

[joa-quim]

While I like the examples in two tabs, I'm afraid I don't like clear the mix of classic and modern in the man pages. I think it is confusing for users. And also, while I'm not really a modern mode user, remember that it was introduced so that new users are advised to use this only. Bring back the classic to front page is a step back on this.

[seisman]

That's a good point. The problem is, there are still many users who use classic mode and there are also many existing classic-mode scripts. So modern-mode users still need to know at least the existence of the classic mode. I think we just need to have a section explaining the differences between classic and modern modes and then link to that section when necessary.