docs: improve main workflow readability (no functional change)

[Sebastianhayashi]

Summary

• add structured comments in .github/workflows/main.yml describing CI reading path and local quick subset commands
• clarify the aggregate all-green gate and canary publish condition
• no workflow behavior changes (comment-only update)

Reading path comparison

Before: readers had to infer gating and execution flow by scanning many jobs. After: a top-level reading path maps from changes to build/test gates in one pass.

Evidence

• queue item: PR_QUEUE_1_2H.md feat: allows undefined collections #3
• target file: https://github.com/payloadcms/payload/blob/main/.github/workflows/main.yml

Local validation

• git diff --name-only confirms only .github/workflows/main.yml
• npx --yes prettier --check .github/workflows/main.yml

Risk

• low risk: readability only
• no functional change