Implement Marp docs adapter generation

[Gujiassh]

Summary

• replace the Marp docs adapter placeholder with a real local Marp CLI generation path
• support pptx/pdf/html generation from markdown via stdin, including theme, custom binary, output directory, and local-file options
• preserve dry-run behavior and surface Marp stderr when generation fails

Validation

• npx vitest run packages/docs/marp/src/index.test.ts
• isolated TypeScript check for packages/docs/marp/src/index.ts
• git diff --check

Notes

• Full workspace pnpm install/typecheck is currently blocked by an unrelated existing workspace issue: packages/dns/azure depends on @sh1pt/core@workspace:* but the workspace package is named @profullstack/sh1pt-core.