docs(features): add sitemap documentation

Add docs/content/features/sitemap.md documenting the automatic
XML sitemap generation feature:

- Output location and contents
- Auto-generation behavior
- lastmod date handling
- Linking and validation tips

Follows established pattern from feeds.md.

docs/content/features/sitemap.md

@@ -0,0 +1,60 @@
+---
+title: Sitemap
+description: Automatic XML sitemap generation for SEO
+weight: 7
+---
+
+sukr generates an XML sitemap at build time for search engine optimization.
+
+## Output
+
+After building, you'll find `sitemap.xml` in your output directory:
+
+```text
+public/
+├── index.html
+├── feed.xml
+├── sitemap.xml  ← XML sitemap
+└── blog/
+    └── ...
+```
+
+## Sitemap Contents
+
+The sitemap includes URLs for:
+
+- Homepage (`/index.html`)
+- Section index pages (`/blog/index.html`, etc.)
+- All content items within sections
+- Standalone pages (top-level `.md` files)
+
+## Auto-generation
+
+Sitemap generation happens automatically during every build. No configuration required.
+
+URLs use the `base_url` from `site.toml` to construct absolute URLs as required by the sitemap protocol.
+
+## Last Modified Dates
+
+If content has a `date` field in frontmatter, it's included as `<lastmod>`:
+
+```yaml
+---
+title: My Post
+date: 2024-01-15
+---
+```
+
+Content without dates omits the `<lastmod>` element.
+
+## Linking to the Sitemap
+
+Add a link in your `base.html` template or `robots.txt`:
+
+```text
+Sitemap: https://example.com/sitemap.xml
+```
+
+## Validation
+
+Test your sitemap with [Google's Rich Results Test](https://search.google.com/test/rich-results) or the [XML Sitemap Validator](https://www.xml-sitemaps.com/validate-xml-sitemap.html).