build: add a top-level Makefile to drive the build

Some consumers may be interested in building the whole enchilada. Add a top
level Makefile to make that feasible.

Both the documentation and website build can run in parallel, and one only
needs to run either the all or generate targets from the top level.

Reviewed-by:	gjb, hrs
Differential-Revision:	https://reviews.freebsd.org/D28385

Makefile

@@ -0,0 +1,21 @@
+#
+# This file is intended to drive the build of the entire doc tree.  In order to
+# build both the documentation and the website, one only need to execute:
+#
+# make all
+#
+# Here at the top-level of the repository.  The same target may be executed in
+# the individual directories to build just the documentation or just the
+# website.
+#
+# Note that the Makefiles within the individual components may also be used to
+# spin up hugo's internal webserver for testing, by default on port 1313.  This
+# can be done with the `run` target.
+#
+
+SUBDIR+=	documentation
+SUBDIR+=	website
+
+SUBDIR_PARALLEL=	yes
+
+.include <bsd.subdir.mk>