ci: Add docs notification workflow on release

[jayden-lee-sb]

Summary

• Add a GitHub Actions workflow that automatically notifies the sendbird-docs repository whenever a new SDK release is published.
• On each release event, this workflow triggers a repository_dispatch to sendbird-docs, passing the SDK type, release tag, and release URL so that documentation can be updated accordingly.

Test plan

• Verify the workflow file is correctly placed at .github/workflows/notify-docs-update.yml
• Confirm the SDK_TYPE env variable matches this repository
• Publish a test release and verify the docs repo receives the dispatch event

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 10.87%. Comparing base (37e0e35) to head (2e41e48).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #288   +/-   ##
=======================================
  Coverage   10.87%   10.87%           
=======================================
  Files         360      360           
  Lines        9054     9054           
  Branches     2575     2556   -19     
=======================================
  Hits          985      985           
- Misses       7993     8068   +75     
+ Partials       76        1   -75     

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[OnestarLee]

lgtm!