[nrf fromtree] doc: fix PDF build issue due to extlink references

LaTeX code generated by Sphinx causes problems when using extlinks in
references. Mitigate the problem by just using the extlink role
directly.

Signed-off-by: Gerard Marull-Paretas <[email protected]>
(cherry picked from commit d253847)

Author: gmarull

doc/getting_started/installation_win.rst

@@ -16,7 +16,7 @@ command-prompt. This allows you to use software such as the :ref:`Zephyr SDK
 .. warning::
       Windows 10 version 1803 has an issue that will cause CMake to not work
       properly and is fixed in version 1809 (and later).
-      More information can be found in `Zephyr Issue 10420`_
+      More information can be found in :github:`Zephyr Issue 10420 <10420>`.
 
 #. `Install the Windows Subsystem for Linux (WSL)`_.
 
@@ -33,4 +33,3 @@ command-prompt. This allows you to use software such as the :ref:`Zephyr SDK
    the documentation itself here.
 
 .. _Install the Windows Subsystem for Linux (WSL): https://msdn.microsoft.com/en-us/commandline/wsl/install_guide
-.. _Zephyr Issue 10420: :github:`10420`

doc/guides/west/why.rst

@@ -117,7 +117,4 @@ West is:
   interoperability with third party tools, and means Zephyr developers
   can always find out what is happening "under the hood" when using west.
 
-See `Zephyr issue #6205`_ and for more details and discussion.
-
-.. _Zephyr issue #6205:
-   :github:`6205`
+See :github:`Zephyr issue #6205 <6205>` and for more details and discussion.