feat(docs,blog,pages): add support for "unlisted" front matter - hide md content in production (#8004)

Co-authored-by: sebastienlorber <lorber.sebastien@gmail.com>

packages/docusaurus-utils/src/contentVisibilityUtils.ts

@@ -0,0 +1,54 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+type Env = 'production' | 'development';
+
+/**
+ * To easily work on draft/unlisted in dev mode, use this env variable!
+ * SIMULATE_PRODUCTION_VISIBILITY=true yarn start:website
+ */
+const simulateProductionVisibility =
+  process.env.SIMULATE_PRODUCTION_VISIBILITY === 'true';
+
+/**
+ * draft/unlisted is a production-only concept
+ * In dev it is ignored and all content files are included
+ */
+function isProduction(env: Env | undefined): boolean {
+  return (
+    simulateProductionVisibility ||
+    (env ?? process.env.NODE_ENV) === 'production'
+  );
+}
+
+/**
+ * A draft content will not be included in the production build
+ */
+export function isDraft({
+  frontMatter,
+  env,
+}: {
+  frontMatter: {draft?: boolean};
+  env?: Env;
+}): boolean {
+  return (isProduction(env) && frontMatter.draft) ?? false;
+}
+
+/**
+ * An unlisted content will be included in the production build, but hidden.
+ * It is excluded from sitemap, has noIndex, does not appear in lists etc...
+ * Only users having the link can find it.
+ */
+export function isUnlisted({
+  frontMatter,
+  env,
+}: {
+  frontMatter: {unlisted?: boolean};
+  env?: Env;
+}): boolean {
+  return (isProduction(env) && frontMatter.unlisted) ?? false;
+}