doc: add a new doc/ directory, and move two markdown docs into them

I figure sooneror later we'll have more of these docs, hence let's give
them a clean place to be.

This leaves NEWS and README/README.md as well as the LICENSE texts in
the root directory of the project since that appears to be customary for
Free Software projects.

src/basic/verbs.c

@@ -41,7 +41,7 @@ bool running_in_chroot_or_offline(void) {
         /* Added to support use cases like rpm-ostree, where from %post scripts we only want to execute "preset", but
          * not "start"/"restart" for example.
          *
-         * See ENVIRONMENT.md for docs.
+         * See doc/ENVIRONMENT.md for docs.
          */
         r = getenv_bool("SYSTEMD_OFFLINE");
         if (r < 0 && r != -ENXIO)