docs: add RELEASING.md for manual tag-and-notes releases #339

Job	Run time
mypy	25s
ruff + pip-audit (ubuntu-latest)	27s
API integration tests + coverage (ubuntu-latest)	24s
Frontend unit tests (vitest) (ubuntu-latest)	27s
API integration tests + coverage (windows-latest)	59s
Frontend unit tests (vitest) (windows-latest)	1m 57s
ruff + pip-audit (windows-latest)	58s
Frontend unit tests (vitest) (macos-latest)	43s
Verify prod requirements.txt is sufficient	8s
ruff + pip-audit (macos-latest)	19s
Performance benchmarks (gated)	37s
pytest (macos-latest)	58s
API integration tests + coverage (macos-latest)	26s
pytest (windows-latest)	1m 47s
pytest (ubuntu-latest)	54s
	11m 29s

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