doc: Check redirect entries are valid

While it's by definition not possible to check that the "source" for a
redirect entry is a valid path, the "destination" could be checked to be
pointing to an existing resource. This will also help catch potential
errors in the "source" as well since it's common to mess up the paths
for both.

Fixes #77797

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>