The PCR GUI Inserts extension lets you easily add pieces of HTML code at several useful places of the GUI.
See also the CHANGELOG.md file provided with the code.
(1) Obtain the code from GitHub
(2) Extract the files in a directory called PCRGUIInserts in your extensions/ folder.
(3) Add the following code at the bottom of your "LocalSettings.php" file:
require_once "$IP/extensions/PCRGUIInserts/PCRGUIInserts.php";
(4) Configure this extension as needed for your wiki. See the documentation.
(5) Go to "Special:Version" on your wiki to verify that the extension is successfully installed.
(6) Done.