docs: add RELEASING.md

[MikeGoldsmith]

Summary

Adds RELEASING.md covering the per-package release model this repo uses.

Contents:

• Frames the independent (non-coordinated) release model — each package owns its own version, CHANGELOG.md, and cadence.
• End-to-end manual release steps: build the towncrier release section, bump the version, tag, build via scripts/build_a_package.sh, twine upload, GitHub release, bump to next dev version.
• Notes the static ## Unreleased entries that pre-date towncrier and need manual folding on the first release.
• Lists explicitly what's still missing: PyPI names per package, prepare-release / package-release workflows, backport workflow, patch-release tooling. All tracked in Bootstrap opentelemetry-python-genai repo #15.

Merge ordering

Important

Depends on #16 (towncrier setup). The changelog steps in this document reference towncrier, which is added to the dev dependencies and configured per package in #16. Do not merge this PR until #16 lands. Kept as a draft for that reason.

Notes

• Skip Changelog label applied (docs-only).

[Copilot]

Pull request overview

Adds release-process documentation for the repository’s independent per-package release model, including manual release steps while automated release workflows are still pending.

Changes:

• Adds RELEASING.md with publishable package list and release flow.
• Documents changelog, version bump, tagging/building, PyPI upload, GitHub release, and next-dev-version steps.
• Captures missing release infrastructure tracked by #15.

Comments suppressed due to low confidence (3)

RELEASING.md:98

• The release-notes extraction command does not extract the release body. In awk range patterns, the end pattern ^## also matches the starting ## Version <version> line, so the range ends on the same line and sed '$d' removes the only line. Maintainers following this command would create a GitHub release with empty notes.

  --notes-file <(awk '/^## Version <version>/,/^## /{print}' <package>/CHANGELOG.md | sed '$d')

RELEASING.md:76

• This uses <pypi-name> as PACKAGE_NAME, but scripts/build_a_package.sh uses PACKAGE_NAME to locate **/$pkg_name/pyproject.toml. The document also says the PyPI names will differ from the current package directories, so the documented command will fail unless the directory/project name matches the PyPI name or the build script is updated.

PACKAGE_NAME=<pypi-name> VERSION=<version> ./scripts/build_a_package.sh

RELEASING.md:120

• The stated unstable version pattern does not match several current package versions (for example, anthropic/langchain/weaviate use 2.0b0.dev, google-genai uses 2.5b0.dev, and openai-agents-v2 uses 0.2.0.dev). Following this section as written could cause maintainers to bump releases to the wrong version scheme.

- **Unstable components** (everything currently in this repo): versions look
  like `0.Yb0.dev` on `main`, `0.Yb0` at release time, then bump to
  `0.{Y+1}b0.dev`.