[SECURITY] Several issues with page on deactivating Directory Index for Apache

[sypets]

See also issue #1990 and pending PR #1991

More issues:

• should the page exist as all, since it is not TYPO3 specific? Should general Webserver security recommendations bloat up the guide or should we link elsewhere for that?
• the directive already exists in the standard .htaccess file
• the instructions for the other webservers (Nginx etc.) is missing
• only necessary if mod_authindex is active

Will address some of this in open PR