[EPS-1397] Add engineering onboarding guide with diagrams

[marcelo-xavier-hf]

Summary

Adds a diagram-rich onboarding guide for engineers joining the team that owns this repo. The guide lives at docs/onboarding.md and is grounded in concrete file paths so a new mid/senior IC can ramp up without paging through the codebase.

The doc covers:

• Overview & Purpose — the repo is a build/publish wrapper around upstream grpc/grpc, no source of its own.
• Tech Stack — Bazel via Bazelisk v1.7.5, GitHub Actions, the linux@amd64 + macos@amd64 matrix.
• Architecture — Mermaid diagrams: high-level pipeline, dependency view, the release-flow sequence (per matrix entry), and the distribution path.
• Key Modules — step-by-step walk through .github/workflows/release.yml, the README.md, and the LICENSE.
• Data Flow — what happens between "Maintainer creates release" and "tarballs land on the release page."
• Local Dev Setup — both editing-the-workflow and reproducing-a-build-locally paths.
• Testing — current state (no tests) plus practical confidence checks.
• Deployment — the release procedure, common failure modes, and history (ARM PR Feature/pccs 11868 add arm support #6).

Documentation-only — no code changes.

Test plan

• docs/onboarding.md is the only file in the diff.
• Every relative file path referenced in the doc resolves to an existing file in the repo (workflow, README, LICENSE).
• All Mermaid blocks use supported syntax (flowchart / sequenceDiagram).
• Reviewer eyes the rendered Mermaid in the GitHub UI for any syntax surprises.

🤖 Generated with Claude Code

[marcelo-xavier-hf]

Repo not owned by GenAI enablement team.