feat: add extend_post_content.html hook partials for post sections

**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: ```html {{- 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](https://github.com/adityatelange/hugo-PaperMod/wiki/Translations#want-to-add-your-language-). - [x] I have enabled [maintainer edits for this PR](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/allowing-changes-to-a-pull-request-branch-created-from-a-fork). - [x] 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. - [x] This change **does not** include any CDN resources/links. - [x] 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. --- **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.