T8615: switch yaml-language-server schema URL to JSON-schema

Replaces the human-docs file-format URL (which serves HTML and cannot
drive editor validation) with the machine-readable JSON-schema URL
that yaml-language-server actually resolves for autocomplete and
schema validation.

Old: https://docs.mergify.com/configuration/file-format/
New: https://docs.mergify.com/mergify-configuration-schema.json

Phase 0 CR: 0
0 finding(s).

Refs: T8615 (re-scoped 2026-05-28), IS-421

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