[RFC] Collection of learning materials for Crystal

[straight-shoota]

In the Crystal Book we have several learning materials: a structured tutorial and guides on specific topics. They are easily accessible from the Book landing page. However, there are also many good 3rd party resources that we're currently not linking anywhere.
It would be useful, especially for newcomers, to have a curated list of learning materials about Crystal.
I had already started collecting some ideas on #98, but this really deserves a separate issue.

As an example, these are some extensive courses on Crystal:

• https://exercism.org/tracks/crystal
• https://grox.io/language/crystal/course
• https://codingphase.teachable.com/p/crystal-lang-programming-language-master-course

Other materials.

• An Introduction to the Crystal Programming Language
• Crystal Programming (video tutorial)
• Crystal Lang Tutorial:Installing Crystal Programming Language (video series)
• A Quick look at Crystal
• Creating a Cryptocurrency in the Crystal Programming Language
• Crystal for Rubyists

The list can of course also included other kinds of resources, such as "hardware" books etc.

There are many different categories for learning materials, so I think it would make sense to establish some kind of taxonomy. The details are to be determined.
The materials should be curated to ensure a minimum level of quality and relevance. For example, it would be useless for learners if they're presented with 10 different blog posts about how to write Hello World. We ought to pick the best ones (that's subjective, of course, but what can you do...).
Evaluating the quality of non-publicly available resource could be difficult. But I'm sure we can ask providers of such materials for access in return for endorsement.
Accessibility restrictions should be recorded in the list, of course.

An important metric regarding quality is also how up to date a resource is. For example, an introduction to Crystal 1.0 would still probably be mostly accurate for the latest Crystal release (1.4), but readers will miss out on recent additions. For older versions, there will likely be even some incompatibilities. So it might be a good idea to record the Crystal version each resource is targeting (and keep tracking that when it upgrades).

I'm writing this issue on crystal-website, but I'm not decided about the final location. It could be part of this repo, or https://github.com/crystal-lang/crystal-book. Maybe even a stand-alone resource similar to https://github.com/veelenga/awesome-crystal (that could still be integrated into the website, of course).