Some <h3> elements do not contain an anchor

[rozsazoltan]

Typically, Markdown headings like #, ##, and ### are used, and their compiled HTML versions contain the appropriate link references.

./src/docs/theme.mdx

#### Relationship to variants

<h4 id="relationship-to-variants">
  <a href="#relationship-to-variants" class="anchor">Relationship to variants</a>
</h4>

However, in the case of custom h1, h2, h3, etc. elements, this property is not present by default.

./src/docs/functions-and-directives.mdx

<h3 id="import-directive">@import</h3>

<h3 id="import-directive">
  @import
</h3>

It's a bit confusing that there is an ID pointing to the heading, but it's harder to copy because clicking on the heading doesn't navigate to itself.