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

layouts/llms.txt

@@ -0,0 +1,36 @@
+# {{ .Site.Title }}
+
+> {{ .Site.Params.description }}
+
+{{ range $section := site.Sections }}
+{{- template "llms-section-tree" dict "context" . "level" 2 }}
+{{ end }}
+
+{{- $rootPages := where site.RegularPages "Section" "" }}
+{{- if $rootPages }}
+
+## Root Pages
+{{- range $rootPages }}
+- [{{ .Title }}]({{ .Permalink }}): {{ .Summary | plainify | truncate 100 | strings.TrimSpace }}{{ if .Date }} - Published {{ .Date.Format "2006-01-02" }}{{ end }}
+{{- end }}
+{{- end }}
+
+---
+Generated on {{ now.Format "2006-01-02 15:04:05 UTC" }}
+Site: {{ .Site.BaseURL }}
+
+{{- define "llms-section-tree" -}}
+{{- $context := .context -}}
+{{- $level := .level | default 2 -}}
+{{- $headerHashes := strings.Repeat $level "#" -}}
+{{- "\n" -}}
+{{ $headerHashes }} {{ $context.Title }}
+
+{{- range $context.RegularPages }}
+- [{{ .Title }}]({{ .Permalink }}): {{ .Summary | plainify | truncate 100 | strings.TrimSpace }}{{ if .Date }} - Published {{ .Date.Format "2006-01-02" }}{{ end }}
+{{- end }}
+
+{{- range $context.Sections }}
+{{ template "llms-section-tree" dict "context" . "level" (add $level 1) }}
+{{- end }}
+{{- end -}}