Add the [params] concept to front matter

This is deliberately very simple, but should not break anything. We need to introduce this in baby steps, but this should allow us to introduce this in the documentation.

Note that the `params` section's key/values will be added to `.Params` last. This means that you can have different values for "Hugo's summary" and the custom ".Params.summary" if you want to.

Updates #11055

hugolib/page__meta.go

@@ -421,9 +421,20 @@ func (p *pageState) setMetaPostParams() error {
 	var sitemapSet bool
 
 	var draft, published, isCJKLanguage *bool
+	var userParams map[string]any
 	for k, v := range pm.params {
 		loki := strings.ToLower(k)
 
+		if loki == "params" {
+			vv, err := maps.ToStringMapE(v)
+			if err != nil {
+				return err
+			}
+			userParams = vv
+			delete(pm.params, k)
+			continue
+		}
+
 		if loki == "published" { // Intentionally undocumented
 			vv, err := cast.ToBoolE(v)
 			if err == nil {
@@ -589,6 +600,10 @@ func (p *pageState) setMetaPostParams() error {
 		}
 	}
 
+	for k, v := range userParams {
+		pm.params[strings.ToLower(k)] = v
+	}
+
 	if !sitemapSet {
 		pm.sitemap = p.s.conf.Sitemap
 	}