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