Allow disabling the default RouteVoter via configuration

[ousamabenyounes]

Fix #382

Registering the built-in RouteVoter is currently unconditional. Applications that supply their own performance-optimized voter still pay for it: the default RouteVoter is called for every menu item whenever a custom voter returns null (because it cannot decide).

This adds a route_voter boolean option (default true, so behavior is unchanged) that lets you stop the bundle from registering the default RouteVoter:

knp_menu:
    route_voter: false

When false, the knp_menu.voter.router service definition is removed.

Docs (docs/index.rst config reference, all three formats) and the XSD schema were updated in the same change.

Test verification (RED → GREEN)

New tests: KnpMenuExtensionTest::testDisableRouteVoter (+ default now asserts the voter is registered) and the XSD ConfigurationTest case now carries route-voter.

RED — on unmodified master (production change reverted, new tests only):

1) ...KnpMenuExtensionTest::testDisableRouteVoter
Symfony\Component\Config\Definition\Exception\InvalidConfigurationException:
Unrecognized option "route_voter" under "knp_menu".

2) ...ConfigurationTest::testConfigurationMatchesXsd#1
Line 1: attribute 'route-voter': The attribute 'route-voter' is not allowed.

Tests: 4, Assertions: 8, Errors: 1, Failures: 1.

GREEN — with the fix:

OK, but there were issues!
Tests: 36, Assertions: 76, PHPUnit Notices: 15, Skipped: 1.

Baseline master is Tests: 35 with the same 15 notices / 1 skipped — this PR adds one test and introduces no new failures. composer validate --strict and PHP-CS-Fixer (--dry-run) are both clean.