doc: media type handlers (#689)

* reference: media type handlers

* Use function instead of computed field in editable task

---------

Co-authored-by: Laurence Isla <lau.isla.c@gmail.com>

docs/references/api/media_type_handlers.rst

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+Media Type Handlers
+###################
+
+PostgREST offers builtin handlers for common media types such as ``application/json`` and ``text/csv``. You can add handlers for other media types or override the builtin handlers.
+
+Standard Media Types Handlers
+=============================
+
+Builtins handlers are provided for the following standard media types:
+
+* ``application/json``
+* ``text/csv``
+* ``application/geo+json``
+* ``application/x-www-form-urlencoded``
+* ``*/*``, uses the same handler as ``application/json``.
+
+Vendor Media Types Handlers
+===========================
+
+PostgREST also includes its own vendored media types, handlers for these are provided but cannot be overridden.
+
+* ``application/vnd.pgrst.object``
+* ``application/vnd.pgrst.array``
+* ``application/vnd.pgrst.plan``
+
+Custom Media Type Handlers
+==========================
+
+TODO