help: Add "Add a link preview" documentation.

This is a server setting so I created the section "Server settings" in the help sidebar for this to go under, and rewrote the copy and retook the images that were originally done by @Privisus due to some issues.

templates/zerver/help/add-a-link-preview.md

@@ -0,0 +1,28 @@
+# Add a link preview
+
+{!server-admin-only.md!}
+
+Link previews are a way to see an overview of a link's content before
+clicking on it. Previews can provide a useful summary of the content of a site
+before you click on it, while also providing a quick integrity check to ensure
+the link you meant to post is in fact correct.
+
+![Link preview example](/static/images/help/link-preview.png)
+
+Follow these steps to enable link previews:
+
+1. Open the `zproject/settings.py` file in the Zulip directory.
+    !!! tip ""
+        To learn more about the settings and how to configure
+        them, take a look at [this documentation](https://zulip.readthedocs.io/en/latest/subsystems/settings.html)
+
+2. Search for the line with the attribute `INLINE_URL_EMBED_PREVIEW` and set the
+   value from `False` to `True` to enable the embed preview feature.
+   ![Setting "INLINE_URL_EMBED_PREVIEW" to true.](/static/images/help/inline-url-true.png)
+
+3. Restart the server to apply the changes.
+
+![Multiple link preview example](/static/images/help/link-preview-max.png)
+
+Zulip currently supports up to five link previews per post — only the first
+five link previews in the message will be shown.

templates/zerver/help/include/server-admin-only.md

@@ -0,0 +1,2 @@
+!!! warn ""
+    **Note:** This feature can only be controlled by server administrators.

templates/zerver/help/include/sidebar.md

@@ -35,7 +35,6 @@
 * [Share and upload files](/help/upload-and-share-files)
 * [Manage your uploaded files](/help/manage-your-uploaded-files)
 <!-- Automatically link to an external issue tracker (improve wording) -->
-<!-- Add a link preview -->
 * [Enable pressing Enter to send](/help/enable-or-disable-pressing-enter-to-send)
 * [Verify a message was sent](/help/verify-that-your-message-has-been-successfully-sent)
 <!-- What to do if the server returns an error -->
@@ -137,6 +136,10 @@
 * [Configure authentication methods](/help/configure-authentication-methods)
 * [Add a custom linkification filter](/help/add-a-custom-linkification-filter)
 
+
+## Server settings
+* [Add a link preview](/help/add-a-link-preview)
+
 ## Users & bots
 * [Deactivate or reactivate a user](/help/deactivate-or-reactivate-a-user)
 * [Deactivate or reactivate a bot](/help/deactivate-or-reactivate-a-bot)