Add first lesson for the new module tutorials.

[djacu]

Part of a larger effort captured in #966. This was discussed at length during the last team meeting.

• Migrates the first lesson from https://nixos-modules.nix.みんな/lessons/a-basic-module/lesson/. This was fairly straightforward; minor tweaks to code blocks and admonitions.
• The tone has been left unchanged; it is consistent with the other lessons but inconsistent with other articles on nix.dev.
• No "infra" has been carried over. I.e. the output from the nix and script files is static and not part of the site's build process.