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 <federico.vaga@vaga.pv.it>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
2 files changed