Add other OSS projects to CONTRIBUTING.md

[itstrueitstrueitsrealitsreal]

Context

Fixes #3753

Implementation

Added a button on https://nusmods.com/contribute to link to a newly added portion of open source projects added to CONTRIBUTING.md.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
nusmods-export	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 13, 2024 0:13am
nusmods-website	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 13, 2024 0:13am

[vercel]

@itstrueitstrueitsrealitsreal is attempting to deploy a commit to the modsbot's projects Team on Vercel.

A member of the Team first needs to authorize it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.77%. Comparing base (2d4743d) to head (320082c).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3759      +/-   ##
==========================================
+ Coverage   53.87%   54.77%   +0.89%     
==========================================
  Files         274      274              
  Lines        6032     6043      +11     
  Branches     1449     1452       +3     
==========================================
+ Hits         3250     3310      +60     
+ Misses       2782     2733      -49     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.