Document array values for tsconfig extends

[vivekjm]

Summary

Updates the TSConfig extends reference to document that extends can be either a single string or an array of strings.

The page already hinted at "one or more" in the one-line summary, but the body only described the single-string form. This adds the array behavior, notes that later entries override earlier entries, and includes a short multi-base example.

Fixes microsoft/TypeScript#62915.

Validation

• npx --yes prettier@2.8.8 --check packages/tsconfig-reference/copy/en/options/extends.md
• git diff --check
• Sanity-checked TypeScript config parsing with typescript-for-docs; a temp config using extends: ["./configs/base", "./configs/node"] resolved later target/module values from the second config.

I also tried npx --yes pnpm@10.30.3 run --filter=tsconfig-reference lint, but it failed before linting this page because the local workspace could not resolve the built @typescript/twoslash/dist/index.js entrypoint. Building @typescript/vfs succeeded; building @typescript/twoslash still failed resolving @typescript/vfs. The environment also reports Node v20.15.0 while the repo requests >=20.19.