[codex] Add the 4.5 contributor runbook

[sidmohan0]

Summary

• add a contributor-facing 4.5 runbook covering Python environments, install profiles, focused/broad tests, docs build, and optional-profile smoke checks
• document the current 4.5 version frame and release-flow boundary, including when the final 4.5.0 bump should happen
• call out that the external German regex PR review happens after the local release-prep baseline
• link the root CONTRIBUTING.md to the new runbook and add it to the Sphinx docs nav

Stack

This PR is stacked on #143, #142, #141, and #139. The diff against dev will include those earlier commits until they merge.

Verification

• DATAFOG_NO_TELEMETRY=1 DO_NOT_TRACK=1 .venv312/bin/python -m sphinx -b html docs docs/_build/html
• .venv312/bin/pre-commit run --files CONTRIBUTING.md docs/index.rst docs/contributing.rst --show-diff-on-failure
• git diff --check
• DATAFOG_NO_TELEMETRY=1 DO_NOT_TRACK=1 .venv312/bin/python -m pytest tests/test_runtime_dependency_safety.py tests/test_no_network_core.py -q

Refs DFPY-73