Update README/docs config examples to current RectorConfig::configure()->withPaths() form

[bbrala]

Companion to GitLab issue #3446874 (reported by joachim).

Rector now auto-generates a rector.php using the fluent API:

return RectorConfig::configure()
    ->withPaths([...]);

But the docs are inconsistent with this:

• The README still shows $rectorConfig->sets([...]).
• The attribute-conversion doc (docs/core_plugin_conversion.md) shows the old return static function(RectorConfig $rectorConfig): void { $rectorConfig->ruleWithConfiguration(...) } style.

Update the README and docs examples to the current RectorConfig::configure() fluent form so new users aren't confused (this contributes to the common "rector runs but produces no diff" reports).