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/index.rst

@@ -20,7 +20,6 @@ Welcome to the Yocto Project Documentation
    Yocto Project Software Overview <https://www.yoctoproject.org/software-overview/>
    Tips and Tricks Wiki <https://wiki.yoctoproject.org/wiki/TipsAndTricks>
 
-
 .. toctree::
    :maxdepth: 1
    :caption: Manuals
@@ -37,6 +36,12 @@ Welcome to the Yocto Project Documentation
    Test Environment Manual <test-manual/index>
    bitbake
 
+.. toctree::
+   :maxdepth: 1
+   :caption: Security
+
+   Yocto Project Security Reference <security-reference/index>
+
 .. toctree::
    :maxdepth: 1
    :caption: Release Manuals