Rewrite landing page on nix.dev

Description

Today the landing page on nix.dev explains that "The Nix ecosystem is a DevOps toolkit to achieve:..." then lists bullets points, some of which are links to documentation pages, others of which are just text.

Problem

The landing page does not explain the role of nix.dev in the documentation ecosystem, does not explain which documentation the users will find on this site, and does not make clear the information architecture of the site.

Approach

The landing page should tell the user the role the site plays e.g. "nix.dev is the official home of ..." or "nix.dev contains official tutorials and recipes for ...".

The landing page should also have sections reflecting the information architecture of the site e.g. tutorials, recipes, reference materials, and concepts. Each section should be present on the landing page and should have a short description of what each documentation category contains as well as some links to example materials found in that category. For example, the "Recipes" section would contain the following description:

In the Recipes section you'll find quick, step-by-step answers for how to accomplish specific tasks such as building Docker images from Nix derivations and setting up binary caches.

NixOS / nix.dev