doc: add a page for downloading the U-Boot logo

Add a page showing the logo with and without text.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/learn/index.rst

@@ -6,4 +6,5 @@ Learn about U-Boot
 .. toctree::
    :maxdepth: 1
 
+   logo
    talks

doc/learn/logo.rst

@@ -0,0 +1,20 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+U-Boot logo
+===========
+
+The project uses the logos below.
+
+Logo with text
+--------------
+
+.. image:: ../../tools/logos/u-boot_logo_with_text.svg
+  :width: 10em
+  :alt: U-Boot logo with text
+
+Logo without text
+-----------------
+
+.. image:: ../../tools/logos/u-boot_logo.svg
+  :width: 10em
+  :alt: U-Boot logo without text