Favorite websites with shortened hyperlinks

Sure, you’re looking for the shortcode API. That would enable you to write a short piece of code that does exactly what you want. As easy as this in your functions.php:

function wpse246274_wiki( $atts ) {
  $link = "http://en.wikipedia.org/wiki/" . $atts['link'];
  return $link;
  }
add_shortcode( 'wikilink', 'wpse246274_wiki' );

Which you would use like this in your post:

<a href="https://wordpress.stackexchange.com/questions/246274/[wikilink link="geography']">Geography</a>

You could even leave out the attribute and just use

<a href="https://wordpress.stackexchange.com/questions/246274/[wikilink]geography">Geography</a>

But that doesn’t look as elegant.