Add a new "Security" section

The current security-related documentation is a bit hard to find and
hidden within the development manual. However these are processes that
are not part of a development task but is rather a vulnerability
reporting process.

Create a new "Security" section in the documentation to gather this
information. This will be directly visible in the sidebar when opening
the documentation.

Split the previous security-subjects.rst document into 2 documents:

- security-team.rst: defines the roles of the security teams and its
  members.

- reporting-vulnerabilities.rst: guide to report vulnerabilities to the
  security team.

The plan is to backport these documents to active releases. As a
consequence, this section should be free of instructions and information
that only make sense for a specific release. It should _not_ contain
documents on how to enable security features with Yocto on target
devices, this is unrelated and can be left in the development manual
(for example: dev-manual/vulnerabilities.rst to deal with CVEs).

(From yocto-docs rev: 80556704f8b60b5bf903da497909cfda7dd1b28b)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 81e14ca2d5cff9e2104c556655144b069633790c)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/security-reference/index.rst

@@ -0,0 +1,14 @@
+.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
+
+================================
+Yocto Project Security Reference
+================================
+
+.. toctree::
+   :caption: Table of Contents
+   :numbered:
+
+   security-team
+   reporting-vulnerabilities
+
+.. include:: /boilerplate.rst