docs: swap config.yaml reference to RST

docs/reference/files/file-config-yaml.rst

@@ -0,0 +1,66 @@
+.. _file-config-yaml:
+.. highlight:: yaml
+
+``config.yaml``
+***************
+
+.. important::
+    Starting with Charmcraft 2.5, this file is created automatically from information
+    you provide in :ref:`charmcraft.yaml<file-charmcraft-yaml>`. For backwards
+    compatibility, Charmcraft will continue to allow the use of this file, but you may
+    not duplicate keys across the two files.
+
+The ``config.yaml`` in a charm's root directory is an optional file that may be used to define the configuration options supported by a charm.
+
+The definitions are collected under a single YAML map called ``options``. The rest of this doc gives details about this map.
+
+``options``
+===========
+
+**Status:** Required if the file exists.
+
+**Purpose:** The ``options`` key allows charm authors to declare the configuration options that they have defined for a charm.
+
+**Structure:** The key contains a definition block for each option, where each definition consists of a charm-author-defined option name and an option description, given in 3 fields -- type, description, and default value:
+
+.. code::
+
+    options:
+      <option name>:
+        default: <default value>
+        description: <description>
+        type: <type>
+      <option name>:
+        default: <default value>
+        description: <description>
+        type: <type>
+      ...
+
+In some cases, it may be awkward or impossible to provide a sensible default. In these cases, ensure that it is noted in the description of the configuration option. It is acceptable to provide ``null`` configuration defaults or omit the ``default`` field.
+
+
+.. dropdown:: Example
+
+    .. code::
+
+        options:
+          name:
+            default: Wiki
+            description: The name, or Title of the Wiki
+            type: string
+          skin:
+            default: vector
+            description: skin for the Wiki
+            type: string
+          logo:
+            default:
+            description: URL to fetch logo from
+            type: string
+          admins:
+            default:
+            description: Comma-separated list of admin users to create: user:pass[,user:pass]+
+            type: string
+          debug:
+            default: false
+            type: boolean
+            description: turn on debugging features of mediawiki