docs(site): rewrite how-it-works on precomputed catchment connections

[CooperBigFoot]

docs(site): rewrite how-it-works on precomputed catchment connections

What changed

Rewrote docs/how-it-works.md around precomputed hydrofabric catchment connections.
Removed the raster/vector/D8 teaching narrative and replaced it with the point-to-watershed lookup, upstream traversal, and merge flow.
Kept the Heberger credit and credits.md link, with refinement reduced to one closing sentence.

Why

The page now matches the intended audience and framing: hydrology users need to understand that shed starts from a hydrofabric whose unit catchments and upstream connections are already computed.
The explanation keeps D8 details out of the main narrative while preserving the optional outlet-trim constraint for datasets that ship the raster.

Verification

1. PASS: /Users/nicolaslazaro/Desktop/work/shed/crates/python/.venv/bin/mkdocs build --strict -f mkdocs.yml -d .mkdocs-site-check && rm -rf .mkdocs-site-check
2. PASS: ! grep -q '—' docs/how-it-works.md
3. PASS: ! grep -qiE 'HDX|TerminalRefinementStrategy|row group|range read|rayon|pyo3|projjson|hilbert|xdg|object.store' docs/how-it-works.md && ! grep -qE '\bArrow\b' docs/how-it-works.md
4. PASS: ! grep -qiE "isn['’]t|it['’]s not|it is not|not just|not original|rather than" docs/how-it-works.md
5. PASS: ! grep -qiE '[0-9]+ ?(ms\b|millisecond|second|minute)' docs/how-it-works.md
6. PASS: ! grep -qiE 'Two classic ways|flow accumulation|eight directions' docs/how-it-works.md
7. PASS: head -n 6 docs/how-it-works.md | grep -qi 'watershed' && ! grep -qi 'drainage basin' docs/how-it-works.md
8. PASS: grep -q 'Heberger' docs/how-it-works.md && grep -q 'credits.md' docs/how-it-works.md && grep -qF 'the whole upstream area that drains to it, the watershed' docs/how-it-works.md && grep -qF 'pre-built river-network files' docs/how-it-works.md
9. PASS: test "$(grep -ci 'refin' docs/how-it-works.md)" -le 1
10. PASS: test "$(git diff --name-only)" = "docs/how-it-works.md"
11. PASS: test "$(git show --name-only --format= HEAD | tr -d '\n')" = "docs/how-it-works.md"