keyboard-shortcuts: Document browse back and forward in history.

Documents in help center `/keyboard-shortcuts` and in the app `?`
menu the shortcuts used by browsers for navigating back and forward
through the open tab's history, which are made to work in Zulip.

Also, updates `adjust_mac_shortcuts` to update the shortcut keys
for users with Mac user agents.

Fixes #18542.

static/js/common.ts

@@ -61,6 +61,7 @@ export function adjust_mac_shortcuts(key_elem_class: string, kbd_elem = true): v
         ["PgUp", "↑"],
         ["PgDn", "↓"],
         ["Ctrl", "⌘"],
+        ["Alt", "⌘"],
     ]);
 
     const fn_shortcuts = new Set(["Home", "End", "PgUp", "PgDn"]);