fix(www): add local KB rewrite URL (#50452)

## What kind of change does this PR introduce?

Bug fix for the local www development environment.

## What is the current behaviour?

Running `pnpm dev:www` fails Next.js rewrite validation because
`NEXT_PUBLIC_KB_URL` is missing from the tracked public environment
defaults, producing an `undefined` destination for `/kb`.

## What is the new behaviour?

The tracked public environment defines the local KB URL, matching
`.env.local.example`, so the www rewrite configuration is valid in a
fresh checkout.

## To test

1. Run `pnpm dev:www`.
2. Open http://localhost:3000.
3. Confirm www starts without the `Invalid rewrites found` error for
`/kb`.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Configuration**
* Added a public URL setting for accessing the knowledge base service in
local development.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/www/.env

@@ -10,3 +10,4 @@ NEXT_PUBLIC_STUDIO_URL="http://localhost:8082"
 NEXT_PUBLIC_REFERENCE_DOCS_URL="http://localhost:3010"
 NEXT_PUBLIC_LIBRARY_URL="http://localhost:3004"
 NEXT_PUBLIC_DESIGN_SYSTEM_URL="http://localhost:3003"
+NEXT_PUBLIC_KB_URL="http://localhost:3008/kb"