Misc permalinks adjustments

* Move config loading to the page package
* Fix a lower bound panic for the `:sections` slice syntax.
* Always return the `:title`
* Add some permalinks integration tests
* Also see issues below

Fixes #9448
Fixes #11184
See #8523

config/allconfig/alldecoders.go

@@ -206,49 +206,9 @@ var allDecoderSetups = map[string]decodeWeight{
 	"permalinks": {
 		key: "permalinks",
 		decode: func(d decodeWeight, p decodeConfig) error {
-			p.c.Permalinks = make(map[string]map[string]string)
-
-			p.c.Permalinks["page"] = make(map[string]string)
-			p.c.Permalinks["section"] = make(map[string]string)
-			p.c.Permalinks["taxonomy"] = make(map[string]string)
-			p.c.Permalinks["term"] = make(map[string]string)
-
-			config := maps.CleanConfigStringMap(p.p.GetStringMap(d.key))
-			for k, v := range config {
-				switch v := v.(type) {
-				case string:
-					// [permalinks]
-					//   key = '...'
-
-					// To sucessfully be backward compatible, "default" patterns need to be set for both page and term
-					p.c.Permalinks["page"][k] = v
-					p.c.Permalinks["term"][k] = v
-
-				case maps.Params:
-					// [permalinks.key]
-					//   xyz = ???
-
-					if (k == "page") || (k == "section") || (k == "taxonomy") || (k == "term") {
-						// TODO: warn if we overwrite an already set value
-						for k2, v2 := range v {
-							switch v2 := v2.(type) {
-							case string:
-								p.c.Permalinks[k][k2] = v2
-
-							default:
-								return fmt.Errorf("permalinks configuration invalid: unknown value %q for key %q for kind %q", v2, k2, k)
-							}
-						}
-					} else {
-						return fmt.Errorf("permalinks configuration only allows per-kind configuration 'page', 'section', 'taxonomy' and 'term'; unknown kind: %q", k)
-					}
-
-				default:
-					return fmt.Errorf("permalinks configuration invalid: unknown value %q for key %q", v, k)
-				}
-			}
-
-			return nil
+			var err error
+			p.c.Permalinks, err = page.DecodePermalinksConfig(p.p.GetStringMap(d.key))
+			return err
 		},
 	},
 	"sitemap": {