Document course deadline calendar feeds

The course platform now exposes a per-course iCalendar feed for deadlines:

https://courses.datatalks.club//calendar.ics

We should document how students can use it.

Suggested docs content:

• Explain that each course has a calendar feed with homework deadlines, project submission deadlines, and project peer review deadlines.
• Explain that the feed is generated from the current course data, so deadline changes appear when the calendar app refreshes the subscription.
• Mention that Google Calendar refreshes subscribed calendars on its own schedule, so updates are not instant.
• Add steps for Google Calendar:
  1. Open Google Calendar.
  2. Go to Other calendars.
  3. Choose From URL.
  4. Paste the course calendar.ics URL.
  5. Add the calendar.
• Mention that Apple Calendar, Outlook, and other calendar apps can subscribe to the same URL as an iCalendar feed.
• Add a short example using a real course slug once the target docs page is chosen.

Relevant platform issue: DataTalksClub/course-management-platform#74

Start by locating the target docs page and checking nearby guidance for course resources or calendar integrations. Add the iCalendar feed explanation, Google Calendar steps, notes about refresh timing and other calendar apps, and a real course-slug example; it is done when students can subscribe and understand how deadline updates appear.