docs: add an Undici 7 to 8 migration guide

[mcollina]

Summary

• add a migration guide for upgrading applications and libraries from Undici 7 to 8
• cover the main upgrade points: Node.js version requirements, dispatcher handler API changes, onBodySent() / onRequestSent(), HTTP/2 defaults, Blob validation, and global dispatcher notes
• add the guide to the docs sidebar

Why

Undici 8 includes a few behavior and API changes that are easy to miss when upgrading from Undici 7.

This guide gives users a single place to check the main migration steps instead of piecing them together from release notes and API docs.

Testing

• npm run lint

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.93%. Comparing base (dc4351a) to head (3448b2a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4963   +/-   ##
=======================================
  Coverage   92.92%   92.93%           
=======================================
  Files         111      111           
  Lines       35831    35831           
=======================================
+ Hits        33297    33298    +1     
+ Misses       2534     2533    -1     

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