social preview: Move into a social directory

This reduces clutter from the logo directory. Only the logo is now part
of that directory.

Also remove the PNG files. These are build outputs.

social/README.md

@@ -0,0 +1,16 @@
+NixOS assets for social media
+=============================
+
+The files in this directory are the source files used for different
+"social media" usage.
+
+
+GitHub repository preview
+-------------------------
+
+ - nix-social-preview.svg
+ - nixos-social-preview.svg
+
+These files are used in the Nix and Nixpkgs repositories to serve as a
+preview for social media "cards" (from different social media networks) when
+linking to a GitHub repository from the NixOS project.