add sitemap override example, closes #120

Author: timvink

Diff for: docs/howto/override-a-theme.md

@@ -143,4 +143,28 @@ We can use the [mkdocs template variables](https://www.mkdocs.org/dev-guide/them
         </div>
     {% endif %}
     {% endblock %}
-    ```
+    ```
+
+## Example: Populate `sitemap.xlm`
+
+Having a correct lastmod in your `sitemap.xlm` is important for SEO, as it indicates to Search engines when to re-index pages, see [this blog from Bing](https://blogs.bing.com/webmaster/february-2023/The-Importance-of-Setting-the-lastmod-Tag-in-Your-Sitemap).
+
+[`@thesuperzapper`](https://github.com/thesuperzapper) shared this [override](https://squidfunk.github.io/mkdocs-material/customization/?h=overri#extending-the-theme) in [mkdocs-git-revision-date-localized-plugin#120](https://github.com/timvink/mkdocs-git-revision-date-localized-plugin/issues/120):
+
+```html
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+{%- for file in pages -%}
+    {% if not file.page.is_link and (file.page.abs_url or file.page.canonical_url) %}
+    <url>
+         <loc>{% if file.page.canonical_url %}{{ file.page.canonical_url|e }}{% else %}{{ file.page.abs_url|e }}{% endif %}</loc>
+         {#- NOTE: we exclude `lastmod` for pages using a template, as their update time is not correctly detected #}
+         {%- if not file.page.meta.template and file.page.meta.git_revision_date_localized_raw_iso_datetime %}
+         <lastmod>{{ (file.page.meta.git_revision_date_localized_raw_iso_datetime + "+00:00") | replace(" ", "T") }}</lastmod>
+         {%- endif %}
+         <changefreq>daily</changefreq>
+    </url>
+    {%- endif -%}
+{% endfor %}
+</urlset>
+```