doc: fix build on Windows with Meson 0.58

doc: fix build on Windows with Meson 0.58

The `doc` target used `echo` as its command.
On Windows, `echo` is always a shell built-in, there is no binary.
Starting from meson 0.58, `run_target()` always searches for command
executable and no longer accepts `echo` as such on Windows.
Replace plain `echo` with a Python one-liner.

Fixes: d02a2dab2dfb ("doc: support building HTML guides with meson")
Cc: stable@dpdk.org
Reported-by: Rob Scheepens <rob.scheepens@nutanix.com>
Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Luca Boccassi <bluca@debian.org>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>