Remove Liam's talk from the overview page

[seisman]

Description of proposed changes

The overview page lists conference presentations about the development of PyGMT, but Liam's talk is a tutorial. I think we should remove it from the list.

[maxrjones]

I actually find it non-intuitive to find the presentations in the overview section and would prefer moving all the presentations to the external resources page. Liam's talk could still go under tutorials rather than presentations.

[seisman]

I actually find it non-intuitive to find the presentations in the overview section and would prefer moving all the presentations to the external resources page.

Then the overview page would be too short 🤦

[maxrjones]

I actually find it non-intuitive to find the presentations in the overview section and would prefer moving all the presentations to the external resources page.

Then the overview page would be too short 🤦

I have ideas for other content. But this is a nice concise PR so we can go ahead with this and discuss in the overview page more generally in a separate issue.