fix: update install custom packages tutorial to use the v2 build system

[dobrac]

Note

Rewrites the custom packages guide to use the v2 SDK-based template build and updates runtime install examples and Sandbox usage.

• Docs — Quickstart (docs/quickstart/install-custom-packages.mdx):
  • Switch to v2 SDK template flow:
    • Replace CLI/Dockerfile-based steps with SDK-driven templates (Template().fromTemplate(...)) in TS/Python.
    • Add build scripts (build.prod.ts/build.prod.py) using Template.build with alias/resources and build logs.
    • Show building via npx tsx build.prod.ts or python build.prod.py.
  • Usage updates:
    • Start sandboxes by alias (e.g., Sandbox.create("custom-packages")).
    • Update examples to await Sandbox.create() where needed and modernize code snippets.
  • Cleanup/Removals:
    • Remove E2B CLI login/init steps and Dockerfile-based package installation instructions.

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