Document course deadline calendar feeds

[alexeygrigorev]

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