docs: add a single HFLOW_* environment variable reference page

[chintondutta]

Fixes #9.

The gap

HFlow recognizes seven HFLOW_* environment variables, documented across five different places with no single reference -- and HFLOW_TEST_BUCKET_URL wasn't documented anywhere at all (grepped HFLOW_ under src/ and tests/ to enumerate authoritatively; the issue listed six, there are seven).

Change

Adds docs/ENVIRONMENT.md: one table (variable, purpose, default, where it applies), linked to the existing pages that own each variable's operational detail rather than duplicating it. Linked from docs/README.md's Reference section, per the documentation contract (one primary purpose per page, linked from the index).

Left the existing per-topic docs (README.md Requirements, RUNTIME.md, CATALOG.md, CONTRIBUTING.md, generated DEPLOY.md) untouched -- this page is the lookup table, not a replacement for where each variable is explained in context.

Testing

docker run --rm -v "$PWD:/data" -w /data lycheeverse/lychee:latest --no-progress --include-fragments \
  --exclude '^https://github\.com/Hebbian-Robotics/hflow/(issues|security/advisories/new)$' \
  --exclude-path references/mcap-spec.md \
  --exclude-path references/foxglove-CompressedVideo.proto .
# 237 Total, 0 Errors

uv run pytest -q       # 297 passed, 3 skipped; unrelated: tests/test_ffmpeg.py fails/errors in this sandbox (no ffmpeg/ffprobe on PATH)
uv run ruff check --fix
uv run ruff format
uv run ty check

(lychee wasn't on PATH in this sandbox; ran it via the official Docker image instead of installing.)

[github-actions]

👋 Hi @chintondutta — thanks for the contribution! To keep starter issues available
for other contributors and give every pull request a real review, we accept
1 open pull request per contributor at a time.

You already have #52 open, so this one is being closed automatically.
Once your open pull request is merged or closed, feel free to reopen this one —
no work is lost.