Improve code block highlighting

[JuliaEvseeva]

This PR brings the improvement for the block highlighting.

Screen.Recording.2025-12-18.at.15.35.33.mov

To highlight only the text without highlighting the entire line with background, use the following highlight shortcode:

{{< highlight class="hl-text-only" lang="text" params="hl_lines=10 19" >}}
// Code.
{{< /highlight >}}

Where:

• class="hl-text-only" – the class that is used in the code theme to apply the appropriate styles.
• lang – the code language.
• hl_lines=10 19 – the lines that should be highlighted.

To highlight the full line, use:

{{< highlight lang="text" params="hl_lines=10 19" >}}
// Code.
{{< /highlight >}}

or

```text {hl_lines=[10,19]}
// Code.
```

The full guide will be added in the AUTHORING.md.

Also, this PR increases the required Hugo version that will also be updated in the main website. It is needed to fix the TOC rendering if the headings contains links.