docs: Introduce deprecated APIs list

As discussed in the "API replacement/deprecation" thread[1], this makes
an effort to document what things shouldn't get (re)added to the kernel,
by introducing Documentation/process/deprecated.rst.

[1] https://lists.linuxfoundation.org/pipermail/ksummit-discuss/2018-September/005282.html



Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>