Add docs for patching an instance

Signed-off-by: Joas Schilling <coding@schilljs.com>

admin_manual/issues/applying_patch.rst

@@ -0,0 +1,47 @@
+==================
+Patching Nextcloud
+==================
+
+Applying a patch
+----------------
+
+Patching server
+^^^^^^^^^^^^^^^
+
+1. Navigate into your Nextcloud server's root directory (contains the ``status.php`` file)
+2. Now apply the patch with the following command::
+
+    patch -p 1 < /path/to/the/file.patch
+
+.. note::
+
+   There can be errors about not found files, especially when you take a patch from GitHub there might be development or test files included in the patch. when the files are in build/ or a tests/ subdirectory it is mostly being
+
+Patching apps
+^^^^^^^^^^^^^
+
+1. Navigate to the root of this app (mostly ``apps/[APPID]/``), if you can not find the app there use the ``sudo -u www-data php occ app:getpath APPID`` command to find the path.
+2. Now apply the patch with the same command as in `Patching server`_
+
+Reverting a patch
+-----------------
+
+1. Navigate to the directory where you applied the patch.
+2. Now revert the patch with the ``-R`` option::
+
+    patch -R -p 1 < /path/to/the/file.patch
+
+Getting a patch from a GitHub pull request
+------------------------------------------
+
+If you found a related pull request on GitHub that solves your issue, or you want to help developers and verify a fix works, you can get a patch for the pull request.
+
+1. Using https://github.com/nextcloud/server/pull/26396 as an example.
+2. Append ``.patch`` to the URL: https://github.com/nextcloud/server/pull/26396.patch
+3. Download the patch to your server and follow the `Applying a patch`_ steps.
+4. In case you are on an older version, you might first need to go the the correct version of the patch.
+
+.. image:: images/getting-a-patch-from-github.png
+   :alt: backportbot-nextcloud linking to the pull request for an older version.
+
+5. You can find it by looking for a link by the ``backportbot-nextcloud`` or a developer will leave a manual comment about the backport to an older Nextcloud version. For the example above you the pull request for Nextcloud 21 is at https://github.com/nextcloud/server/pull/26406 and the patch at https://github.com/nextcloud/server/pull/26406.patch

admin_manual/issues/index.rst

@@ -1,10 +1,11 @@
 ==========================
 Issues and troubleshooting
-========================== 
+==========================
 
 .. toctree::
     :maxdepth: 2
 
     general_troubleshooting
+    applying_patch
     code_signing
-    
+