docs: add Spacer reference notebook

[Kkkakania]

Summary

Add the missing reference notebook for Spacer, HSpacer, and VSpacer. The page explains fixed and responsive spacing and includes runnable examples for fixed gaps, horizontal distribution, vertical distribution, and bottom-right alignment.

This keeps the related spacer variants together in one discoverable reference page and completes the missing layout entry requested in both issues.

Closes #3415.
Closes #3416.

Validation

• PYTHONPATH=. .venv/bin/python -m pytest --nbval-lax examples/reference/layouts/Spacer.ipynb -q (5 passed)
• uvx pre-commit run --files examples/reference/layouts/Spacer.ipynb

---

AI disclosure: this documentation contribution was AI-assisted. I reviewed every example, executed the notebook top-to-bottom, and ran the repository pre-commit hooks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.33%. Comparing base (b2fb6b8) to head (be78d4e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8681      +/-   ##
==========================================
- Coverage   85.89%   85.33%   -0.56%     
==========================================
  Files         350      350              
  Lines       57169    57169              
==========================================
- Hits        49104    48784     -320     
- Misses       8065     8385     +320     

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[Kkkakania]

CI note: the failed UI jobs are unrelated existing timing failures in Tabulator horizontal-rescroll, reload/JupyterLite, and Jupyter server tests; none touches the new Spacer reference notebook. The type job reports only the existing coroutine callback annotations in panel/io/document.py and panel/io/server.py. Pre-commit, core, and all unit matrices pass.

[philippjfr]

Thank you @Kkkakania!