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>

Documentation/index.rst

@@ -22,10 +22,7 @@ The following describes the license of the Linux kernel source code
 (GPLv2), how to properly mark the license of individual files in the source
 tree, as well as links to the full license text.
 
-.. toctree::
-   :maxdepth: 2
-
-   process/license-rules.rst
+* :ref:`kernel_licensing`
 
 User-oriented documentation
 ---------------------------

Documentation/process/howto.rst

@@ -57,12 +57,13 @@ of doing things.
 Legal Issues
 ------------
 
-The Linux kernel source code is released under the GPL.  Please see the
-file, COPYING, in the main directory of the source tree, for details on
-the license.  If you have further questions about the license, please
-contact a lawyer, and do not ask on the Linux kernel mailing list.  The
-people on the mailing lists are not lawyers, and you should not rely on
-their statements on legal matters.
+The Linux kernel source code is released under the GPL.  Please see the file
+COPYING in the main directory of the source tree. The Linux kernel licensing
+rules and how to use `SPDX <https://spdx.org/>`_ identifiers in source code are
+descibed in :ref:`Documentation/process/license-rules.rst <kernel_licensing>`.
+If you have further questions about the license, please contact a lawyer, and do
+not ask on the Linux kernel mailing list.  The people on the mailing lists are
+not lawyers, and you should not rely on their statements on legal matters.
 
 For common questions and answers about the GPL, please see:
 

Documentation/process/index.rst

@@ -19,6 +19,7 @@ Below are the essential guides that every developer should read.
 .. toctree::
    :maxdepth: 1
 
+   license-rules
    howto
    code-of-conflict
    development-process

Documentation/process/license-rules.rst

 .. SPDX-License-Identifier: GPL-2.0
 
+.. _kernel_licensing:
+
 Linux kernel licensing rules
 ============================