docs: explain three-step release process

[rmi22186]

Summary

• document branch and track selection for all three release workflows
• explain the Step 1 release tag handoff to Steps 2 and 3
• clarify dry-run behavior, atomic synchronization, and required master/main workflow parity

Test plan

• Verify documentation against current workflow guards and branch mappings
• Run git diff --check

[cursor]

PR Summary

Low Risk
Documentation-only README change; no code, workflow, or release-control logic is modified.

Overview
Adds maintainer docs for the three-step staging release process, covering separate v2 (staging/master) and v3 (v3-staging/main) tracks.

Explains how to run staging-step-1.yml (npm publish), optional staging-step-2.yml (release-order branches), and staging-step-3.yml (atomic CDN/branch sync), including dryRun behavior, exact Step 1 tag handoff, and that the three workflow files must stay identical on master and main before a real release.

^{Reviewed by Cursor Bugbot for commit 3d07881. Bugbot is set up for automated code reviews on this repo. Configure here.}

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mparticle-automation]

🎉 This PR is included in version 2.79.3 🎉

The release is available on:

• v2.79.3
• GitHub release

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