Add curated list of OpenRefine tutorials recommended for new contributors

We often have new contributors coming in who don't know anything at all about OpenRefine as a user. They might be coming to the project because they want to gather experience contributing to an open source project (for instance: Outreachy/GSoC applicants), or because we hired them to work in a particular capacity in the project (for instance: hiring a designer, a security consultant, a community manager…)

To onboard those new contributors on the project, it would be helpful to have a pre-defined set of resources we can point them at, to learn how to use OpenRefine. This knowledge of the tool from a user perspective is generally very useful to get involved in the project.

We currently list a few resources in passing in https://openrefine.org/docs/technical-reference/code-contributions but it would be helpful to have a bigger list, more visible (also advertised to other types of contributors, not just developers), where people would be able to find tutorials focusing on certain topics.

Start with the existing resources in the technical-reference/code-contributions documentation linked in the issue, then inspect how the site organizes contributor and user-facing pages. Curate tutorials by topic and make the list visible to non-developer contributors; done means new contributors can find a clear, useful set of OpenRefine learning resources.