fix: use site regular pages for related content lookup (#1073)

Closes #1072

This PR fixes an issue where related content is not displayed in the
Ananke theme unless `toc` is enabled.

The current template uses:

```go-html-template
{{- $related_content := .RegularPages.Related . -}}
```

This causes the related content lookup to fail in some page contexts,
resulting in no related pages being rendered even when related content
exists.

This PR changes the lookup to:

```go-html-template
{{- $related_content := .Site.RegularPages.Related . -}}
```

Using `.Site.RegularPages` ensures that related content is searched
across all regular pages in the site, restoring the expected related
content behavior.

The change is limited to the template lookup scope and does not
introduce any new functionality.