chore: Significantly restructure folder layout

This moves the various projects from "type-based" folders (such as
"services" or "tools") into more appropriate semantic folders (such as
"nix", "ops" or "web").

Deprecated projects (nixcon-demo & gotest) which only existed for
testing/demonstration purposes have been removed.

(Note: *all* builds are broken with this commit)

ops/infra/nixos/README.md

@@ -0,0 +1,23 @@
+NixOS configuration
+===================
+
+My NixOS configuration! It configures most of the packages I require
+on my systems, sets up Emacs the way I need and does a bunch of other
+interesting things.
+
+In contrast with earlier versions of this configuration, the Nix
+channel versions are now pinned in Nix (see the beginning of
+[packages.nix][]).
+
+Machine-local configuration is kept in files with the naming scheme
+`$hostname-configuration.nix` and **must** be symlinked to
+`local-configuration.nix` before the first configuration run.
+
+I'm publishing this repository (and my [emacs configuration][]) as a
+convenience for myself, but also as a resource that people looking for
+example Nix or Emacs configurations can browse through.
+
+Feel free to ping me with any questions you might have.
+
+[packages.nix]: packages.nix
+[emacs configuration]: https://github.com/tazjin/emacs.d