docs: move config files + runtime dir doc to new configuration/preface

docs/configuration/preface.rst

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+
+.. _configuration_preface:
+
+=======
+Preface
+=======
+
+-----------------------
+Configuration File Path
+-----------------------
+
+zrepl searches for its main configuration file in the following locations (in that order):
+
+* If set, the location specified via the global ``--config`` flag
+* ``/etc/zrepl/zrepl.yml``
+* ``/usr/local/etc/zrepl/zrepl.yml``
+
+The examples in the :ref:`tutorial` or the ``cmd/sampleconf`` directory should provide a good starting point.
+
+-------------------
+Runtime Directories
+-------------------
+
+zrepl requires runtime directories for various UNIX sockets --- they are documented in the :ref:`config file<conf-runtime-directories>`.
+Your package maintainer / init script should take care of creating them.
+Alternatively, for default settings, the following should to the trick.
+
+::
+
+    mkdir -p /var/run/zrepl/stdinserver
+    chmod -R 0700 /var/run/zrepl
+
+