Update rspress theme callstack to latest

[mdjastrzebski]

Summary

Updates the rspress website to use the @callstack/rspress-theme package. This involves adding the theme dependency, configuring rspress.config.ts to use it, and removing the no-longer-needed custom theme override.

Test plan

1. Navigate to the website directory.
2. Run yarn install to install new dependencies.
3. Run yarn dev to start the development server.
4. Open the local development server in your browser and verify that the new Callstack theme is applied correctly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.34%. Comparing base (203bac9) to head (cee2a89).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1792    +/-   ##
========================================
  Coverage   95.34%   95.34%            
========================================
  Files          94       94            
  Lines        5244     5244            
  Branches      573      893   +320     
========================================
  Hits         5000     5000            
  Misses        244      244            

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.