Deprecate kind, lang, and path from front matter

These were added to the page meta object when we implemented "pages from data", but were not meant to be used in front matter.

That is not supported, so we might as well add validation.

Fixes #12484

hugolib/page_test.go

@@ -1942,3 +1942,29 @@ Hugo: h-Home|
 `,
 	)
 }
+
+// See #12484
+func TestPageFrontMatterDeprecatePathKindLang(t *testing.T) {
+	// This cannot be parallel as it depends on output from the global logger.
+
+	files := `
+-- hugo.toml --
+disableKinds = ["taxonomy", "term", "home", "section"]
+-- content/p1.md --
+---
+title: "p1"
+kind: "page"
+lang: "en"
+path: "mypath"
+---
+-- layouts/_default/single.html --
+Title: {{ .Title }}
+`
+	b := Test(t, files, TestOptWarn())
+	b.AssertFileContent("public/mypath/index.html", "p1")
+	b.AssertLogContains(
+		"deprecated: kind in front matter was deprecated",
+		"deprecated: lang in front matter was deprecated",
+		"deprecated: path in front matter was deprecated",
+	)
+}