Document tsconfig extends arrays

[vivekjm]

Summary

• document that tsconfig.json extends accepts either a string or an array of config paths
• clarify that multiple extended configs are applied in array order, with later configs overriding earlier ones
• add a small array example to the extends TSConfig reference page

Fixes microsoft/TypeScript#62915.

Validation

• git diff --check
• pnpm exec prettier --check packages/tsconfig-reference/copy/en/options/extends.md
• pnpm run --filter=tsconfig-reference lint was attempted after pnpm install --frozen-lockfile; it is blocked locally by the existing workspace build for @typescript/twoslash failing to resolve @typescript/vfs before linting this markdown change. Retried with Node v20.20.0 to satisfy the repo engine requirement and saw the same workspace resolution failure.