user docs: Add documentation for /todo.

Fixes #20141.

Co-authored-by: Alya Abbott <alya@zulip.com>

templates/zerver/help/format-your-message-using-markdown.md

@@ -19,6 +19,7 @@ to allow you to easily format your messages.
 * [Status messages](#status-messages)
 * [Global times](#global-times)
 * [Tables](#tables)
+* [To-do lists](#to-do-lists)
 * [Paragraphs and lines](#paragraphs-and-lines)
 
 ## Emphasis
@@ -245,6 +246,19 @@ The header separators (`---`) must be at least three dashes long.
 
 ![Markdown table](/static/images/help/markdown-table.png)
 
+## To-do lists
+
+Sending a message with the text `/todo` creates a simple collaborative
+to-do list. Any user who can access the message can add tasks by
+entering the task's title and description and clicking "Add task". Once
+created, task titles and descriptions cannot be edited.
+
+Tasks can be marked (and unmarked) as completed by clicking the
+checkboxes on the left.
+
+![Markdown todo-lists](/static/images/help/markdown-todo.png)
+
+
 ## Paragraphs and lines
 
 ```