[Renovate Centralization] Phase 1: Define per-repo override pattern

[farhan]

Context

Part of the Phase 1 discovery for epic #541 — Centralize Renovate config for openedx repos.

Task

Define how individual repos can opt out or extend the shared preset without forking the entire config.

Questions to answer

• How does a repo add custom packageRules (e.g. custom grouping, extra reviewer teams) on top of the shared preset?
• How does a repo opt out of a specific setting (e.g. disable automerge for a package)?
• How does a repo add a manager the shared preset doesn't cover (e.g. Docker, pip)?
• What is the minimum boilerplate a migrated repo's renovate.json should contain?

Expected final shape for a migrated repo

{
  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
  "extends": ["github>openedx/renovate-config:frontend"],
  "packageRules": [
    // repo-specific overrides only
  ]
}

Output

A documented override pattern with 2–3 worked examples (custom grouping, reviewer team, opt-out) included in the design doc.

References

• Parent epic: Centralize Renovate config for openedx repos (frontend + backend) #541
• Renovate extends docs: https://docs.renovatebot.com/config-presets/#preset-hosting