build/doc: add dependencies to 'output' rule

Make the docs depend on all of the public header files and Markdown
files.

Without this, the docs never get rebuilt without a manual 'make clean'.

While we're here, add '@:' to the 'all' rule to silence it.

Change-Id: I304a1bb30464d2a0a60a1c117b7be1f7e543e15c
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/399954


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>