feat: add extend_post_content.html hook partials for post sections

[henrypham67]

What does this PR change? What problem does it solve?

Adds an extend_post_content.html hook partial at a key injection point in single.html, after the post body and before the footer.

Currently, injecting custom content into single.html (e.g., quizzes, flashcard widgets, custom sections after post body) requires copying the entire layouts/_default/single.html into the project as a full override. This creates an ongoing maintenance burden — every upstream change to single.html requires a manual diff and re-merge.

This PR adds a single no-op stub partial that users can override in their own layouts/partials/ without touching the base layout:

{{- partial "extend_post_content.html" . }}

The stub ships empty (comments only), so existing sites are completely unaffected. This follows the same pattern already established by extend_head.html and extend_footer.html in PaperMod.

Was the change discussed in an issue or in the Discussions before?

Yes. Closes #1845

PR Checklist

• This change adds/updates translations and I have used the template present here.
• I have enabled maintainer edits for this PR.
• I have verified that the code works as described/as intended.
• This change adds a Social Icon which has a permissive license to use it.
• This change does not include any CDN resources/links.
• This change does not include any unrelated scripts such as bash and python scripts.
• This change updates the overridden internal templates from HUGO's repository.

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Commit quality assessment: The commit is solid — minimal, focused, zero-breaking-changes, and consistent with PaperMod's existing extend_head.html / extend_footer.html pattern. The stub file comments clearly explain the purpose and override mechanism. Good to go.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[adityatelange]

use comments.html?

[henrypham67]

Thanks for the suggestion! I considered using comments.html as a hook, but it felt like the wrong trade-off — PaperMod only renders it when comments: true is set in frontmatter, which would be semantically misleading for my use case (flashcards/quiz content). It'd also create a collision point if real comment support (Disqus, utterances, etc.) is added later.
The current single.html override is minimal (two if blocks) and follows standard Hugo layout extension patterns. Keeping the concern in its own partial is cleaner than repurposing a comments hook for unrelated UI. Happy to discuss further if you see a downside I'm missing!

[adityatelange]

it is not about how minimal the change is but about how many extend templates we can add for each template which user is not wiling to override for their own convenience.