docs: Add documentation for weekly digest emails.

Added documentation explaining the digest emails feature and how to
disable it, along with a link to the /digest.

Resolves: #14136

templates/zerver/help/digest-emails.md

@@ -0,0 +1,27 @@
+# Weekly digest emails
+
+{!admin-only.md!}
+
+Zulip has a beta feature to send weekly emails to users who haven't
+been active for 5 or more days.  These emails include details on new
+streams created and highlights of traffic (of subscribed streams) that
+can intrigue users.
+
+This feature is disabled by default, but an organization administrator
+can enable it for their organization.  Individual users can opt-out in
+organizations that have enabled it.
+
+You can view a sample digest email for your account in HTML and
+plain-text formats by visiting `https://zulip.example.com/digest/`,
+if `https://zulip.example.com` is your Zulip server URL.
+
+## Enable digest emails for an organization
+
+{start_tabs}
+
+{settings_tab|organization-settings}
+
+1. Under **Notifications**, toggle
+   **Send weekly digest emails to inactive users**.
+
+{end_tabs}

templates/zerver/help/include/sidebar_index.md

@@ -127,6 +127,7 @@
 * [Manage editing of old messages](/help/configure-message-editing-and-deletion)
 * [Hide message content in emails](/help/hide-message-content-in-emails)
 * [Community topic edits](/help/community-topic-edits)
+* [Weekly digest emails](/help/digest-emails)
 * [Disable welcome emails](/help/disable-welcome-emails)
 * [Configure notification bot](/help/configure-notification-bot)
 * [Require topics in stream messages](/help/require-topics)