docs(release): add 4.2.0-beta.1 and 4.2.0 notes; update changelog

[WRRicht3r]

Summary

Add release documentation to prepare a 4.2.0-beta.1 and subsequent 4.2.0 stable
rollout:

• New files: docs/releases/4.2.0-beta.md, docs/releases/4.2.0.md
• New/updated: CHANGELOG.md entries for 4.2.0-beta.1 and 4.2.0
• No runtime code changes; this PR is documentation only

Related Issue

Relates to #1236 (Reown migration), #1235 (toolchain upgrades), #1239 (SDK alignment
docs/CI), #1240 (CI hardening)

Type of Change

• feat (new feature)
• fix (bug fix)
• refactor (no functional change)
• chore/build (tooling, CI, deps)
• docs (README/AGENTS, comments)

Screenshots / Videos

N/A — documentation only; no UI changes.

Test Plan

Commands run locally:
./gradlew detektAll
./gradlew runTest
./gradlew :app:lint
Additional checks and scenarios covered:

• Verified baseline tag (3.7.2) and summarized commits since
• Confirmed post-merge checks (postMergeVerify) ordering via --dry-run
• Alignment print sanity: ./gradlew printPolkadotSdkAlignment

Risks & Rollout

• No code changes in this PR; low risk
• Documentation prepares the path for:
  • Beta: 4.2.0-beta.1 (internal/closed testing)
  • Stable: 4.2.0 staged rollout
• Follow-up PR(s) will bump versionName/versionCode and tag releases

Checklist

• Linked an issue and added a clear description
• Added/updated tests for changed code (where applicable) — N/A (docs only)
• Updated docs (README/AGENTS) when behavior or commands changed — new release
  docs + changelog
• Ran detektAll, runTest, and :app:lint locally (or via CI)
• No secrets or local.properties committed