To do that only on the front page you could use the inlineJS plugin and the following code:-
jQuery(document).bind("contextmenu", function (e) {
e.preventDefault();
alert("To copy the content of this page:-\n\tSave page\n\tPrint page\n\tprint screen\n\tuse wget or CURL\n\thighlight content and press Ctrl+c\n\tView source and download images using direct links\n\nTo re-enable right click - copy and paste into the URL bar:-\njavascript:void(document.oncontextmenu=null);");
});
Important - using that plugin is a potential security risk.
Note also that disabling right-click does not stop people copying your web page.
To disable right-click for the entire DokuWiki, without the same risks as using the plugin - put that code into $DokuWiki/conf/userscript.js