Add release follow-up PR action

[lukewilliamboswell]

Summary

• add create-followup-pr composite action for staged release follow-up branches
• implement GitHub bot commit, deterministic branch push, and PR create/update mechanics
• document the docs/examples follow-up convention and cover the helper with unit tests

Validation

• python3 -m unittest discover -s tests
• /private/tmp/release-package-venv/bin/python scripts/lint_action_shell.py
• /private/tmp/actionlint
• /private/tmp/actionlint tests/fixtures/composite-actions.yml

[lukewilliamboswell]

This was tested using https://github.com/lukewilliamboswell/roc-parser