Link to saas-mediawiki configuration

Summary:
MediaWiki reads configuration from LocalSettings.php.

This configuration is provided by rMWSAAS.

If there is no particular logic like allow to select a directory
according the production or development version, it's possible
to replace this PHP entry point by a symbolic link.

Ref T845.

Test Plan:
php -S … in /srv/mediawiki to check MediaWiki runs
/var/51-wwwroot/saas-mediawiki/LocalSettings.php.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T845

Differential Revision: https://devcentral.nasqueron.org/D1441