Docs: Move xz_pipe_comp.c and xz_pipe_decomp.c to doc/examples_old.

It is good to keep these around to so that if someone has
copied the decompressor bug from xz_pipe_decomp.c he has
an example how to easily fix it.

diff --git a/doc/examples/xz_pipe_comp.c b/doc/examples_old/xz_pipe_comp.c
similarity index 100%
rename from doc/examples/xz_pipe_comp.c
rename to doc/examples_old/xz_pipe_comp.c

diff --git a/doc/examples/xz_pipe_decomp.c b/doc/examples_old/xz_pipe_decomp.c
similarity index 100%
rename from doc/examples/xz_pipe_decomp.c
rename to doc/examples_old/xz_pipe_decomp.c