Add Call-To-Action (CTA) buttons to all cards

[awindsr]

Currently, the cards under Community, Mentors, Interest Groups, Roadmaps, Challenges, Opportunities only display text and visuals.
They don’t provide any action path for the user (e.g., Join, Explore, Apply, Start).
This makes the content feel like placeholder/dummy data.

Tasks

• Add contextual CTAs for each card.
  • Community → “Join Now” → links to signup page
  • Mentors → “Find a Mentor” → links to mentorship directory
  • Interest Groups → “Explore IGs” → links to IG landing page
  • Roadmaps → “View Roadmaps” → links to roadmap section
  • Challenges → “Start a Challenge” → links to challenges section
  • Opportunities → “See Opportunities” → links to jobs/gigs

Acceptance Criteria

• Each card includes a CTA button/link
• CTAs redirect to relevant functional pages
• Buttons styled consistently with µLearn design system

[shrvansudhakara]

@awindsr Hey, I'd like to work on this issue. Pls assign.