I was missing a bit of information at first, considering I have disabled TinyMCE, but author @GitHub was more than helpful and fast with responding.
If you have disabled TinyMCE, like me, this is the way to insert shortcode manually:
The visual editor inserts a shortcode [sta_anchor id="summary" /] (or, if you encapsule text [sta_anchor id="summary"]your text[/sta_anchor]). The shortcode then gets rendered in frontend as <span id="summary" class="sta-anchor " aria-hidden="true"></span>. Either should work, but generally the idea was to provide an easy way to add anchors with the formatting icon in TinyMCE.