How do I configure mediawiki to show heading numbering/lettering in the document?

There is a user-preference to make all headings use outline numbering, but no way to make that a default for all users. Here's a few lines of code which can be added to your LocalSettings.php file which do that.

$wgExtensionFunctions[] = 'wfNumberHeadings';
function wfNumberHeadings() {
    global $wgUser;
    $wgUser->setOption('numberheadings', true);
}

According to this page, you can also change in LocalSettings.php to make this behavior the default.

$wgDefaultUserOptions['numberheadings'] = 1;