feat(llms): add option to exclude pages and sections from llms.txt (#918)

* feat: improve section filtering in llms.txt

* fix(llms): correct where syntax and add page exclusion filter

- Fix invalid Hugo template syntax in where clause
- Use boolean comparison instead of string for llms param
- Add filtering to recursive llms-section-tree template
- Document llms exclusion feature in all languages (en, ja, fa, zh-cn)

---------

Co-authored-by: Xin <xin@imfing.com>

layouts/llms.txt

@@ -1,14 +1,13 @@
 # {{ .Site.Title }}
 
-> {{ .Site.Params.description }}
+> {{ .Site.Params.description | default "" }}
 
-{{ range $section := site.Sections }}
+{{ range where site.Sections "Params.llms" "ne" false }}
 {{- template "llms-section-tree" dict "context" . "level" 2 }}
 {{ end }}
 
-{{- $rootPages := where site.RegularPages "Section" "" }}
+{{- $rootPages := where (where site.RegularPages "Section" "") "Params.llms" "ne" false }}
 {{- if $rootPages }}
-
 ## Root Pages
 {{- range $rootPages }}
 - [{{ .Title }}]({{ .Permalink }}): {{ .Summary | plainify | truncate 100 | strings.TrimSpace }}{{ if .Date }} - Published {{ .Date.Format "2006-01-02" }}{{ end }}
@@ -26,11 +25,11 @@ Site: {{ .Site.BaseURL }}
 {{- "\n" -}}
 {{ $headerHashes }} {{ $context.Title }}
 
-{{- range $context.RegularPages }}
+{{- range where $context.RegularPages "Params.llms" "ne" false }}
 - [{{ .Title }}]({{ .Permalink }}): {{ .Summary | plainify | truncate 100 | strings.TrimSpace }}{{ if .Date }} - Published {{ .Date.Format "2006-01-02" }}{{ end }}
 {{- end }}
 
-{{- range $context.Sections }}
+{{- range where $context.Sections "Params.llms" "ne" false }}
 {{ template "llms-section-tree" dict "context" . "level" (add $level 1) }}
 {{- end }}
 {{- end -}}