Update Beam docs for rebar3 scaffold generation

[dbrattli]

Summary

• Update Beam getting-started and build-and-run docs to reflect the new rebar3 scaffold auto-generation from Fable#4387
• Replace manual erlc compilation steps with rebar3 compile
• Update output structure to show src/ subdirectories, rebar.config, and .app.src files
• Update run commands to use rebar3 output paths (_build/default/lib/*/ebin)
• Fix OTP version requirement (24 → 25) and add rebar3 as prerequisite

Test plan

• Verified end-to-end with a fresh hello-world project using Fable 5.0.0-rc.3 and Fable.Core 5.0.0-rc.1

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