Complete documentation and tutorial pass

Summary

Reshape the beginner-facing documentation so BacktestingKit reads as one guided learning journey instead of several overlapping entry points.

simplify README.md into a lighter front door
make docs/ONBOARDING.md the canonical markdown tutorial
reduce overlap across docs/GETTING_STARTED.md, docs/CHOOSE_YOUR_SURFACE.md, and docs/INDEX.md
promote and align the existing DocC tutorial track
fix stale onboarding/tutorial examples and add lightweight success checkpoints

one obvious markdown starting point
one obvious DocC/tutorial starting point
early-user docs have distinct roles
onboarding flow feels linear from build to app integration
stale summary.result... example usage is removed from the touched onboarding/tutorial surfaces
verification includes doc scans plus swift build and swift test

Completed the documentation/tutorial pass.

Highlights:

simplified README.md into a lighter front door with a single proof-of-success snippet and clearer routing
made docs/ONBOARDING.md the canonical markdown tutorial
reduced overlap across docs/GETTING_STARTED.md, docs/CHOOSE_YOUR_SURFACE.md, and docs/INDEX.md
promoted and aligned the existing DocC tutorial track
fixed stale summary.result... snippets in the touched onboarding/tutorial docs and switched them to summary.metrics...

Verification:

rg -n 'summary\.result\.' README.md docs BacktestingKit/BacktestingKit.docc returned no matches
swift build
swift test passed with 122 tests and 0 failures

Final verification after the last markdown link cleanup:

rg -n 'summary\.result\.' README.md docs BacktestingKit/BacktestingKit.docc returned no matches
swift test passed again with 122 tests and 0 failures

No code-path behavior changed in the final cleanup; the last pass was limited to doc link consistency inside docs/.