Rewrite links in Page.intro with language (#16550)

* Add defaultLanguage behavior

* Add a million logs because I'm confused

* Rewrite intro links

* Undo changes to detect-language

* Add a test

tests/unit/page.js

@@ -90,6 +90,21 @@ describe('Page class', () => {
       expect($(`a[href="/en/${nonEnterpriseDefaultVersion}/articles/about-pull-requests"]`).length).toBeGreaterThan(0)
     })
 
+    test('rewrites links in the intro to include the current language prefix and version', async () => {
+      const page = new Page(opts)
+      page.rawIntro = '[Pull requests](/articles/about-pull-requests)'
+      const context = {
+        page: { version: nonEnterpriseDefaultVersion },
+        currentVersion: nonEnterpriseDefaultVersion,
+        currentPath: '/en/github/collaborating-with-issues-and-pull-requests/about-branches',
+        currentLanguage: 'en'
+      }
+      await page.render(context)
+      const $ = cheerio.load(page.intro)
+      expect($('a[href="/articles/about-pull-requests"]').length).toBe(0)
+      expect($(`a[href="/en/${nonEnterpriseDefaultVersion}/articles/about-pull-requests"]`).length).toBeGreaterThan(0)
+    })
+
     test('does not rewrite links that include deprecated enterprise release numbers', async () => {
       const page = new Page({
         relativePath: 'admin/enterprise-management/migrating-from-github-enterprise-1110x-to-2123.md',