feat: support llms.txt output (#736)

* feat(llms): add llms txt output format

- Introduced a new output format 'llms' in the configuration.
- Updated the example site to utilize the new 'llms' format for the home output.
- Added a new layout file 'llms.txt' for rendering content in the LLMS format.

* docs: add llms.txt support documentation

- Introduced a new section in the configuration guide detailing how to enable the llms.txt output format for improved accessibility to large language models.
- Provided example configuration for adding the 'llms' output format in the site's hugo.yaml.
- Explained the content structure of the generated llms.txt file, including site title, section listings, page summaries, and direct links.

* Update configuration.md

hugo.toml

@@ -0,0 +1,10 @@
+[module]
+  [module.hugoVersion]
+    min = '0.146.0'
+
+[outputFormats]
+  [outputFormats.llms]
+    name= 'llms'
+    baseName = 'llms'
+    mediaType = 'text/plain'
+    isPlainText = true