Link FAQ about indexing in the right-panel footer (#258)

Link FAQ about indexing in the right-panel footer so people can more easily
understand what goes into the result and find issues to track about opting out.

 - #47
 - https://github.com/matrix-org/matrix-public-archive/blob/5caf9dc1b8d5e98fdc99ab06ab9b9d09d2f546ed/docs/faq.md#how-do-i-opt-out-and-keep-my-room-from-being-indexed-by-search-engines

client/css/styles.css

@@ -23,6 +23,20 @@ summary {
   cursor: pointer;
 }
 
+.external-link::after {
+  content: '';
+
+  display: inline-block;
+  width: 1.1ch;
+  height: 1.1ch;
+  margin-left: 0.3ch;
+
+  background: url('../img/external-link-icon.svg');
+  background-repeat: no-repeat;
+  background-size: contain;
+  vertical-align: baseline;
+}
+
 .PrimaryActionButton {
   display: inline-block;
   padding: 4px 16px;

shared/views/RightPanelContentView.js

@@ -2,18 +2,18 @@
 
 const { TemplateView } = require('hydrogen-view-sdk');
 
+const assert = require('matrix-public-archive-shared/lib/assert');
 const CalendarView = require('matrix-public-archive-shared/views/CalendarView');
 const TimeSelectorView = require('matrix-public-archive-shared/views/TimeSelectorView');
-const assert = require('matrix-public-archive-shared/lib/assert');
 
 class RightPanelContentView extends TemplateView {
   render(t, vm) {
     assert(vm.shouldIndex !== undefined);
     assert(vm.shouldShowTimeSelector !== undefined);
 
-    let maybeIndexedMessage = 'This room is not being indexed by search engines.';
+    let maybeIndexedMessage = 'This room is not being indexed by search engines ';
     if (vm.shouldIndex) {
-      maybeIndexedMessage = 'This room is being indexed by search engines.';
+      maybeIndexedMessage = 'This room is being indexed by search engines ';
     }
 
     return t.div(
@@ -33,7 +33,19 @@ class RightPanelContentView extends TemplateView {
             className: 'RightPanelContentView_footer',
           },
           [
-            t.p(maybeIndexedMessage),
+            t.p([
+              maybeIndexedMessage,
+              '(',
+              t.a(
+                {
+                  className: 'external-link RightPanelContentView_footerLink',
+                  href: 'https://github.com/matrix-org/matrix-public-archive/blob/main/docs/faq.md#how-do-i-opt-out-and-keep-my-room-from-being-indexed-by-search-engines',
+                  target: '_blank',
+                },
+                'more info'
+              ),
+              ').',
+            ]),
             t.div(
               {
                 className: 'RightPanelContentView_footerLinkList',