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>

Documentation/process/index.rst

@@ -42,6 +42,7 @@ Other guides to the community that are of interest to most developers are:
    stable-kernel-rules
    submit-checklist
    kernel-docs
+   deprecated
 
 These are some overall technical guides that have been put here for now for
 lack of a better place.