Add a new section "Quicklinks" in the documentation landing page

[seisman]

I propose to add a new section "Quick links" in the documentation landing page, as there are many useful tables or figures, which are hidden in the lengthy cookbook, e.g.:

1. common options
2. projection specifications
3. gmt.conf
4. gmtcolors
5. 35 postscript fonts
6. builtin CPTs
7. octal codes of characters

[seisman]

We already have common options, projection specifications and gmt.conf in the "Reference documentation" section. I think it make more sense to move them to the "Quick links" section.

[PaulWessel]

I agree these are/should be high-traffic areas in the docs. Let's see what @joa-quim thinks before implementing.

[joa-quim]

I agree too and even extend it also list the pen syntax and grid formats (3D nc details)