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

exampleSite/content/docs/guide/configuration.md

@@ -350,6 +350,26 @@ To exclude an entire directory, use the [`cascade`](https://gohugo.io/configurat
 > To block search crawlers, you can make a [`robots.txt` template](https://gohugo.io/templates/robots/).
 > However, `robots.txt` instructions do not necessarily keep a page out of Google search results.
 
+### LLMS.txt Support
+
+To enable [llms.txt](https://llmstxt.org/) output format for your site, which provides a structured text outline for [large language models](https://en.wikipedia.org/wiki/Large_language_model) and AI agents, add the `llms` output format to your site's `hugo.yaml`:
+
+```diff {filename="hugo.yaml"}
+outputs:
+- home: [html]
++ home: [html, llms]
+  page: [html]
+  section: [html, rss]
+```
+
+This will generate an `llms.txt` file at your site's root containing:
+- Site title and description
+- Hierarchical listing of all sections and pages
+- Page summaries and publication dates
+- Direct links to all content
+
+The llms.txt file is automatically generated from your content structure and makes your site more accessible to AI tools and language models for context and reference.
+
 ### Open Graph
 
 To add [Open Graph](https://ogp.me/) metadata to a page, add values in the frontmatter params.

exampleSite/hugo.yaml

@@ -13,9 +13,9 @@ hasCJKLanguage: true
 #     ID: G-MEASUREMENT_ID
 
 outputs:
-  home: [HTML]
-  page: [HTML]
-  section: [HTML, RSS]
+  home: [html, llms]
+  page: [html]
+  section: [html, rss]
 
 defaultContentLanguage: en
 languages: