DFPY-72: Make the v4.5 docs story obvious

[sidmohan0]

Summary

• reshape the docs index so live DataFog 4.5 user docs are the first path
• add a dedicated getting-started page covering the 4.5 core story, install profiles, Python usage, CLI usage, and middleware non-goals
• add a secondary planning/history page that keeps v4/v5 planning and audit artifacts reachable without making them the live entry point
• align the README with the 4.5 lightweight text screening thesis and future middleware boundary

Stack

This PR is stacked on the 4.5 docs/core-path work in #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 README.md docs/index.rst docs/getting-started.rst docs/planning-history.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
• DATAFOG_NO_TELEMETRY=1 DO_NOT_TRACK=1 .venv312/bin/python -m pytest -m "not slow" -q

Refs DFPY-72