Classroom: highlight setup time savings in docs

[cuihtlauac]

Parent issue: #24

Problem

One instructor reported spending five class periods on Python environment setup before adopting devcontainers. With devcontainers, students were productive in 5 minutes. This dramatic contrast is the core value proposition but isn't communicated in the project docs.

Status: Addressed by project design, not yet documented

The image delivers on this promise — everything is pre-built and ready. But the README and docs don't quantify the benefit or contrast it with manual setup.

Supporting evidence from UCSD study (Valstar et al., ITiCSE 2020)

In a 416-student study, 71% of students preferred the DevContainer over pre-configured Linux servers. 70% recommended it to future students, only 8% recommended against.

Key reasons students preferred DevContainers:

• GUI editor preference: The Neovim binary missing from Base Image specification #1 reason was "I like the VSCode editor" — even students who'd used Vim for years preferred VS Code. This validates supporting VS Code as the primary workflow.
• Offline work: Students valued working without internet, unlike SSH servers. The local Docker workflow (.devcontainer-from-scratch/) supports this and should be highlighted.
• Assignment portability: "The DevContainer allows for easy sharing of programming assignments between instructors as the tooling infrastructure is shipped with the assignment." The third-layer architecture already enables this.

Proposed changes

• Add a "Why devcontainers?" section to README highlighting the 5-minute onboarding claim
• Include a comparison: manual OCaml setup (opam init, switch creation, tool installation = 30-60 min) vs devcontainer up (image pull = 2-3 min)
• Highlight offline work capability of the local Docker workflow
• Mention assignment portability as a benefit for instructors