Fix migration documentation id, add redirect for backward-compatibility

[Monteth]

The current "Migrating v3 to v4" doc has an improper ID of migrating-4-to-4, so I fixed that and added a redirect functionality to support links saved somewhere on the web before the change.

BTW we can't test the redirect before the release because it works only in production build 🤷🏻

This plugin is always inactive in development and only active in production because it works on the build output.

https://docusaurus.io/docs/3.5.2/api/plugins/@docusaurus/plugin-client-redirects

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.45%. Comparing base (b137ca0) to head (40ff07a).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1402   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files         175      175           
  Lines        2860     2860           
  Branches      759      759           
=======================================
  Hits         2730     2730           
  Misses         61       61           
  Partials       69       69           

☔ 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.