doc: add some chapter labels

The idea is to make it easier to create references (doc-guide does the same).
This will be used, for example but not only, in translations to point to
the main document.

Signed-off-by: Federico Vaga <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/index.rst

···
    You can adapt this file completely to your liking, but it should at least
    contain the root `toctree` directive.
 
+.. _linux_doc:
+
 The Linux Kernel documentation
 ==============================
 

Documentation/kernel-hacking/index.rst

···
+.. _kernel_hacking:
+
 =====================
 Kernel Hacking Guides
 =====================