Docs: Add $READTHEDOCS_OUTPUT to environment variable reference (#10407)

* Add $READTHEDOCS_OUTPUT to environment variable reference

* Update docs/user/reference/environment-variables.rst

* Update docs/user/reference/environment-variables.rst

Co-authored-by: Manuel Kaufmann <[email protected]>

---------

Co-authored-by: Manuel Kaufmann <[email protected]>

Author: benjaoming

docs/user/reference/environment-variables.rst

@@ -51,6 +51,23 @@ All :doc:`build processes </builds>` have the following environment variables au
 
     :Example: ``/home/docs/checkouts/readthedocs.org/user_builds/project/envs/version``
 
+.. envvar:: READTHEDOCS_OUTPUT
+
+    Base path for well-known output directories. Files in these directories will automatically be found, uploaded and published.
+
+    You need to concatenate an output format to this variable.
+    Currently valid formats are ``html``, ``pdf``, ``htmlzip`` and ``epub``.
+    (e.g. ``$READTHEDOCS_OUTPUT/html/`` or ``$READTHEDOCS_OUTPUT/pdf/``)
+    You also need to create the directory before moving outputs into the destination.
+    You can create it with the following command ``mkdir -p $READTHEDOCS_OUTPUT/html/``.
+    Note that only ``html`` supports multiple files,
+    the other formats should have one and only one file to be uploaded.
+
+    .. seealso::
+
+       :ref:`build-customization:where to put files`
+          Information about using custom commands to generate output that will automatically be published once your build succeeds.
+
 .. envvar:: READTHEDOCS_CANONICAL_URL
 
     Canonical base URL for the version that is built.