ref-manual/structure.rst: document the site.conf file

This was not documented and its usage is going to increase massively
once users start using bitbake-setup. Add a description for this file as
well as an example on how bitbake-setup shares it for multiple builds,
to insist on the "site-wide" usage of this file.

(From yocto-docs rev: 42f68f7de7f0cf74ebdd31fae3173dd3a80386ff)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/structure.rst

@@ -343,6 +343,35 @@ Once the build process gets the sample file, it uses ``sed`` to substitute final
    version of the ``bblayers.conf.sample`` file in the ``meta-poky/conf/templates/default``
    directory.
 
+.. _structure-build-conf-site.conf:
+
+``build/conf/site.conf``
+========================
+
+This configuration file contains the site specific configurations for your build
+environment.
+
+You could for example edit this file to limit the number of threads used by
+:term:`BitBake` (:term:`BB_NUMBER_THREADS`) or set the location from which you
+want to access downloaded files (:term:`DL_DIR`).
+
+This file can be shared for multiple build directories. For example,
+``bitbake-setup`` makes the :ref:`structure-build-conf-site.conf` file a
+symbolic link to a common ``site.conf`` file::
+
+   ├── poky-master-poky-distro_poky-machine_qemux86-64/
+   │   └── build/
+   │       └── conf/
+   │           └── site.conf -> ../../../site.conf
+   ├── poky-master-poky-with-sstate-distro_poky-machine_qemux86-64/
+   │   └── build/
+   │       └── conf/
+   │           └── site.conf -> ../../../site.conf
+   └── site.conf
+
+This way, site-specific settings can be shared for multiple build
+configurations.
+
 .. _structure-build-conf-bblock.conf:
 
 ``build/conf/bblock.conf``