Complete documentation and tutorial pass

[andywkff]

Summary

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

Scope

• 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

Acceptance Criteria

• 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

[andywkff]

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

[andywkff]

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/.