This blog is built with Hugo, and the content is exported from Org files to Hugo-compatible Markdown via ox-hugo. Each heading is followed by an anchor after the export:

## Heading {#heading}
### Another heading {#another-heading}

To make the anchors clickable, Kaushal Modi (the author of ox-hugo) recommends replacing anchors with links (source), the steps are as follows:

First, create a partial layouts/partials/headline-hash.html:

{{ . | replaceRE `(<h[2-9] id="([^"]+)".+)(</h[2-9]+>)` `${1}&nbsp;<a class="headline-hash opacity-0" href="#${2}">#</a> ${3}` | safeHTML }}

Next, apply the partial to .Content:

{{ partial "headline-hash.html" .Content }}

The result will look like this:

<h2 id="heading">Heading&nbsp;<a class="headline-hash" href="#heading">#</a> </h2>
<h3 id="another-heading">Another heading&nbsp;<a class="headline-hash" href="#another-heading">#</a> </h3>

By default, multiple calls of the content_for helper using the same identifier will concatenate and output them together.

Set flush: true to flush all previous content_for calls:

<% content_for :example %>
This block is flushed.
<% end %>

<% content_for :example flush: true do %>
This block will display.
<% end %>