manuals: improve the width of diagrams

Better for EPUB output in particular
- Make some diagrams wider when necessary
- Remove ":align: center" when we have ":width: 100%"
- Update the standards.md files to mention this

(From yocto-docs rev: 848ba7bd8984cc3f4bf6b818259865011cde0476)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/kernel-dev/concepts-appx.rst

@@ -221,7 +221,7 @@ the line-by-line code ``diff`` level is now a trivial operation.
 The following illustration shows the conceptual Yocto Linux kernel.
 
 .. image:: figures/kernel-architecture-overview.png
-   :align: center
+   :width: 100%
 
 In the illustration, the "Kernel.org Branch Point" marks the specific
 spot (or Linux kernel release) from which the Yocto Linux kernel is
@@ -324,6 +324,7 @@ source files used during the build.
 
 .. image:: figures/kernel-overview-2-generic.png
    :align: center
+   :width: 70%
 
 Again, for additional information on the Yocto Project kernel's
 architecture and its branching strategy, see the

documentation/kernel-dev/intro.rst

@@ -106,7 +106,7 @@ modification workflow. The illustration and accompanying list provide
 general information and references for further information.
 
 .. image:: figures/kernel-dev-flow.png
-   :align: center
+   :width: 100%
 
 1. *Set up Your Host Development System to Support Development Using the
    Yocto Project*: See the ":doc:`/dev-manual/start`" section in