Update product page to talk about letters

Letters is now a mature enough feature that we should:
- be raising awareness amongst our users that it’s a thing we offer
- not have letters be a surprise to anyone creating a Notify account for
  the first time

Shouldn’t be merged until:
- [ ] https://github.com/alphagov/notifications-api/pull/1600

tests/app/main/views/test_index.py

@@ -11,8 +11,14 @@ def test_non_logged_in_user_can_see_homepage(
 
     page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
 
-    assert page.select_one('meta[name=description]')['content'].startswith(
-        'GOV.UK Notify lets you send emails and text messages'
+    assert page.h1.text.strip() == (
+        'Send emails, text messages and letters to your users'
+    )
+
+    assert page.select_one('meta[name=description]')['content'].strip() == (
+        'GOV.UK Notify lets you send emails, text messages and letters '
+        'to your users. Try it now if you work in central government, a '
+        'local authority, or the NHS.'
     )