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/migration-guides/release-notes-4.3.rst

@@ -274,7 +274,7 @@ New Features / Enhancements in 4.3
 
    -  New :doc:`../contributor-guide/index` document.
 
-   -  New :doc:`../dev-manual/security-subjects` chapter in the Development
+   -  New "Dealing with Vulnerability Reports" chapter in the Development
       Tasks Manual.
 
    -  Long overdue documentation for the :ref:`ref-classes-devicetree` class.