docs: tidy up TOCs and refs to license-rules.rst

The documentation and TOCs are organized in a manner of a tree. Adding a TOC to
the root, which refers to a file which is located in a subfolder forms a
grid. Those TOCs are a bit confusing and thats why we get additional error
messages while building partial documentation::

  $ make SPHINXDIRS=process htmldocs
  ...
  checking consistency... Documentation/process/license-rules.rst: \
  WARNING: document isn't included in any toctree

To fix it, the *root-license-TOC* is replaced by a reference and the
'license-roles.txt' is added to the Documentation/process/index.rst TOC.

BTW: there was an old licences remark in Documentation/process/howto.rst which
is also updated, mentioning SPDX and pointing to the license-rules.rst

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>