New md files for the four types of docs

[YetAnotherJonWilson]

I added reference.md, tutorials.md, explanation.md and how-to_guides.md, to the root of our repo, to simplify the process of adding resources. I'll try, for now, to manually keep these in sync with the onboarding pod that I created, but soon we'll want to make a decision about using one resource or the other, or somehow automating the process of adding a resource to both (or autmoate syncing them both).

[YetAnotherJonWilson]

I removed these md files, as we decided to use the pod for storing the documentation links, and display the documentation on the frontend of the site, which is currently hosted with Github Pages: https://solid-contrib.github.io/getting-started/.

Also, I believe the issue of potential unreliability of using the pod has been resolved with updates to server-side rendering--if it's working as expected now, when the resources are unavailable, the server should be using a cached version.