feat: live class sessions, CE workflow, and docs (#683)

Adds live-class session scheduling on lessons (join link, date/time,
timezone) with automatic 24h and 1h reminder emails plus a calendar
invite (.ics), a time-change notification with an updated invite, a
teacher-authored welcome email and session invite on enrollment, and a
"Notify students" quiz nudge.

LMS gets an upcoming live-sessions card and Join buttons, plus a
reusable live-session card component (3 states) in @cio/ui with a
Storybook story. The sidebar content tree now reflects sequential
progression locking like the content list, and the course-completion
confetti regression is fixed.

Org-based emails no longer reference ClassroomIO by name. Docs add
live-class, welcome-email, and course-progression guides, expand the
certificates guide (design, signatures, corrected labels), move
plan-required callouts to the top, flatten how-to-guides to the docs
root with redirects, and add screenshots. Includes an add-docs-image
skill.

README.md

@@ -1,5 +1,5 @@
 <a href="https://classroomio.com/">
-  <img alt="ClassroomIO is a no-code tool that allows you build and scale your online bootcamp with ease." src="https://raw.githubusercontent.com/classroomio/classroomio/main/apps/website/static/classroomio-opengraph-image.png" />
+  <img alt="ClassroomIO is a no-code tool that allows you build and scale your online bootcamp with ease." src="https://raw.githubusercontent.com/classroomio/classroomio/main/apps/website/static/classroomio-opengraph-image.jpg" />
   <h1 align="center">ClassroomIO.com</h1>
   <p align="center">
     The Open Source Learning Management System for Companies