This is a simple MediaWiki extension to give you control over the HTML title and meta tags via a tag or parser function.
Follow the instructions at https://www.mediawiki.org/wiki/Composer. The identifier of this extension is tinymighty/wiki-seo.
Prior to using Composer make sure that you remove the code of the extension as well as its invocation from your "LocalSettings.php" file if you already used it before. Composer will automatically load the extension for your wiki.
You can get the extension via Git (specifying WikiSEO as the destination directory):
git clone https://github.com/tinymighty/wiki-seo.git WikiSEO
Or download it as zip archive.
In either case, the "WikiSEO" extension should end up in the "extensions" directory of your MediaWiki installation. If you got the zip archive, you will need to put it into a directory called WikiSEO.
Add the following line to the end of your LocalSettings file:
require_once "$IP/extensions/WikiSEO/WikiSEO.php";
Use this extension as described on the extensions documentation page.