docs(libtofs): explain usage of sub-directory for components

docs/TOFS_pattern.rst

@@ -444,3 +444,74 @@ Resulting in:
 Note: This does *not* override the default value.
 Rather, the value from the pillar/config is prepended to the default.
 
+Using sub-directories for ``components``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If your formula is composed of several components, you may prefer to provides files under sub-directories, like in the `systemd-formula <https://github.com/saltstack-formulas/systemd-formula>`_.
+
+.. code-block::
+
+   /srv/saltstack/systemd-formula/
+     systemd/
+       init.sls
+       libtofs.jinja
+       map.jinja
+       networkd/
+         init.sls
+         files/
+           default/
+             network/
+               99-default.link
+       resolved/
+         init.sls
+         files/
+           default/
+             resolved.conf
+       timesyncd/
+         init.sls
+         files/
+           Arch/
+             resolved.conf
+           Debian/
+             resolved.conf
+           default/
+             resolved.conf
+           Ubuntu/
+             resolved.conf
+
+For example, the following ``formula.component.config`` SLS:
+
+.. code-block:: sls
+
+   {%- from "formula/libtofs.jinja" import files_switch with context %}
+
+   formula configuration file:
+     file.managed:
+       - name: /etc/formula.conf
+       - user: root
+       - group: root
+       - mode: 644
+       - template: jinja
+       - source: {{ files_switch(['formula.conf'],
+                                 lookup='formula'
+                    )
+                 }}
+
+will be rendered on a ``Debian`` minion named ``salt-formula.ci.local`` as:
+
+.. code-block:: sls
+
+   formula configuration file:
+     file.managed:
+       - name: /etc/formula.conf
+       - user: root
+       - group: root
+       - mode: 644
+       - template: jinja
+       - source:
+         - salt://formula/component/files/salt-formula.ci.local/formula.conf
+         - salt://formula/component/files/Debian/formula.conf
+         - salt://formula/component/files/default/formula.conf
+         - salt://formula/files/salt-formula.ci.local/formula.conf
+         - salt://formula/files/Debian/formula.conf
+         - salt://formula/files/default/formula.conf