DOC: Add a central introduction page

[seisman]

Currently, we have many pages that give introductions to GMT, for example:

• https://docs.generic-mapping-tools.org/6.5/tutorial/bash/intro.html
• https://docs.generic-mapping-tools.org/6.5/tutorial/julia/intro.html
• https://docs.generic-mapping-tools.org/6.5/reference/preface.html
• https://docs.generic-mapping-tools.org/6.5/reference/introduction.html

These separate introduction pages make sense in old GMT versions when we provided separate PDF files for the cookbook and tutorial. But now, we no longer provide PDF files and for a single website, we should have a central introduction page.

So I propose to merge the 4 pages into a single introduction page and put it at the start of the website (i.e., before the "Installing GMT" section).

@GenericMappingTools/gmt-docs Thoughts?

[joa-quim]

Hmm, they have not all the same content. The one in the CookBook is lengthier and I think it should stay there. The other three yes, they could be linked from a common one (or even removed in the case of tutorials).