How to generate a link as 'href="#"'?
You could just type it into the template and dont have to do anything clever or twig based? e.g. 

&lt;a href="#"&gt;Show more&lt;/a&gt; 


The core html.html.twig quite happily does this for the #main-content link:

https://api.drupal.org/api/drupal/core%21modules%21system%21templates%21html.html.twig/8.2.x