docs: fix hardcoded link warning messages

Sphinx complains about hardcoded links which can be replaced by an
extlink.

So let's apply its recommendations.

Reported-by: Quentin Schulz <foss+yocto@0leil.net>
(From yocto-docs rev: 47a04ba13d55bd92d18b861e32cf4dba1bf70f84)

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

documentation/bsp-guide/bsp.rst

@@ -166,8 +166,8 @@ section.
 #. *Determine the BSP Layer You Want:* The Yocto Project supports many
    BSPs, which are maintained in their own layers or in layers designed
    to contain several BSPs. To get an idea of machine support through
-   BSP layers, you can look at the `index of
-   machines <&YOCTO_RELEASE_DL_URL;/machines>`__ for the release.
+   BSP layers, you can look at the :yocto_dl:`index of machines
+   </releases/yocto/&DISTRO_REL_TAG;/machines>`
 
 #. *Optionally Clone the meta-intel BSP Layer:* If your hardware is
    based on current Intel CPUs and devices, you can leverage this BSP
@@ -879,7 +879,7 @@ Yocto Project:
    your BSP layer as listed in the ``recipes.txt`` file, which is found
    in ``poky/meta`` directory of the :term:`Source Directory`
    or in the OpenEmbedded-Core Layer (``openembedded-core``) at
-   https://git.openembedded.org/openembedded-core/tree/meta.
+   :oe_git:`/openembedded-core/tree/meta`.
 
    You should place recipes (``*.bb`` files) and recipe modifications
    (``*.bbappend`` files) into ``recipes-*`` subdirectories by

documentation/overview-manual/yp-intro.rst

@@ -221,7 +221,7 @@ your Metadata, the easier it is to cope with future changes.
       possible.
 
    -  Familiarize yourself with the `Yocto Project curated layer
-      index <https://www.yoctoproject.org/software-overview/layers/>`__
+      index :yocto_home:`/software-overview/layers/`
       or the :oe_layerindex:`OpenEmbedded layer index <>`.
       The latter contains more layers but they are less universally
       validated.

documentation/ref-manual/system-requirements.rst

@@ -339,7 +339,7 @@ If you would prefer not to use the ``install-buildtools`` script, you can instea
 download and run a pre-built buildtools installer yourself with the following
 steps:
 
-1. Locate and download the ``*.sh`` at &YOCTO_RELEASE_DL_URL;/buildtools/
+1. Locate and download the ``*.sh`` at :yocto_dl:`/releases/yocto/&DISTRO_REL_TAG;/buildtools/`
 
 2. Execute the installation script. Here is an example for the
    traditional installer: