feat(v2): absolute slugs and slug resolution system (#3084)

* rework slug to allow absolute slugs and slug resolution

* add slug metadata tests

* refactor docs metadata test + fix slug bugs

* fix tests

* fix docs tests failing due to randomness + update snapshot

* add test for addLeadingSlash

packages/docusaurus-plugin-content-docs/src/slug.ts

@@ -0,0 +1,41 @@
+/**
+ * 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.
+ */
+
+import {
+  addLeadingSlash,
+  addTrailingSlash,
+  isValidPathname,
+  resolvePathname,
+} from '@docusaurus/utils';
+
+export default function getSlug({
+  baseID,
+  frontmatterSlug,
+  dirName,
+}: {
+  baseID: string;
+  frontmatterSlug?: string;
+  dirName: string;
+}) {
+  const baseSlug: string = frontmatterSlug || baseID;
+  let slug: string;
+  if (baseSlug.startsWith('/')) {
+    slug = baseSlug;
+  } else {
+    const resolveDirname =
+      dirName === '.' ? '/' : addLeadingSlash(addTrailingSlash(dirName));
+    slug = resolvePathname(baseSlug, resolveDirname);
+  }
+
+  if (!isValidPathname(slug)) {
+    throw new Error(
+      `Unable to resolve valid document slug. Maybe your slug frontmatter is incorrect? Doc id=${baseID} / dirName=${dirName} / frontmatterSlug=${frontmatterSlug} => bad result slug=${slug}`,
+    );
+  }
+
+  return slug;
+}