change tutorials to link to cookbook webpage

[sef43]

as discussed in openmm/openmm-cookbook#14 this changes the tutorials section to link to the tutorials on the cookbook webpage, and adds link to the cookbook and tutorials in the documentation section.

deployment can be seen here: https://sef43.github.io/openmm-org/

[peastman]

I think we should just remove the Tutorials tab. It no longer serves any purpose.

[sef43]

I have removed the tutorials tab

[peastman]

I see you added the cookbook and tutorials to the development docs section. What's the workflow for managing them? They aren't tied to a release in the same way the rest of the documentation is. When do the development versions get built, and when do they get promoted to latest?

[sef43]

Yes that is true, the cookbook has its own release cycle, the main branch of the cookbook is automatically deployed to githubpages as the cookbook/dev/ . It is promoted to latest when a release is done on the cookbook github repo.

We can remove the links to the dev versions from here and just leave the latest versions if you think that is better?

[peastman]

Either way. I just wanted to understand what the intended workflow is. If you want to include the links, that's fine.