docs: add RELEASING.md

[abueide]

Summary

Adds a RELEASING.md to document the release process, which until now was undocumented (the other Segment SDKs each have one — this brings analytics_flutter in line).

The doc was derived from the actual repo state, not a template:

• This is a monorepo of independently-versioned packages published to pub.dev (segment_analytics core + 6 plugins).
• There is no automated release workflow — CI (analytics_flutter.yml) only analyzes and tests. Publishing is done manually with dart pub publish by a verified publisher.
• Plugins depend on core via a pub.dev version constraint (segment_analytics: ^1.1.x), not a path dependency, so the doc calls out the core-before-plugins publish order.
• The core version is declared in two places (pubspec.yaml and lib/version.dart, the latter shipped as context.library.version), so the doc flags keeping them in sync.

Contents

• Package → path → pub.dev table
• SemVer + Conventional Commits versioning, and the core dual-version-file note
• Prerequisites (Flutter SDK, pub.dev publisher auth)
• Step-by-step per-package release: bump → changelog → PR → dart pub publish --dry-run → dart pub publish → tag → verify
• Publish order and "no automated publishing" notes

Notes

• The repo currently has no git tags; I included an optional, package-qualified tagging step (e.g. segment_analytics-vX.Y.Z) as a recommendation for traceability. Happy to drop it if the team prefers not to tag.
• This is purely additive (one new file); no existing docs or code changed.

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