docs: make sure docs are in sync with latest changes

[Azhovan]

What

• Align docs with current behavior for derived snake_case keys and env: matching after sourceenv prefix stripping.
• Update contributor docs to match the current Makefile commands and coverage gate.
• Fix examples/basic docs/config to use the current snake_case keys and working env var names.

Why

The docs and checked-in basic example had drifted from the implementation and were giving incorrect setup/binding guidance.

Type

• Fix
• Feature
• Docs
• Performance
• Breaking change

Testing

Executed:

• go run ./examples/transformer
• env GOCACHE=/tmp/rigging-gocache GOTMPDIR=/tmp go run ./examples/basic
• env GOCACHE=/tmp/rigging-gocache GOTMPDIR=/tmp APP_ENVIRONMENT=production APP_DATABASE__HOST=prod-db.example.com APP_DATABASE__SSL_MODE=require go run ./examples/basic
• env GOCACHE=/tmp/rigging-gocache GOTMPDIR=/tmp APP_FEATURES__ENABLE_METRICS=true APP_FEATURES__RATE_LIMIT=5000 go run ./examples/basic

Would run:

• go test ./...

go run ./examples/transformer
env GOCACHE=/tmp/rigging-gocache GOTMPDIR=/tmp go run ./examples/basic
env GOCACHE=/tmp/rigging-gocache GOTMPDIR=/tmp APP_ENVIRONMENT=production APP_DATABASE__HOST=prod-db.example.com APP_DATABASE__SSL_MODE=require go run ./examples/basic
env GOCACHE=/tmp/rigging-gocache GOTMPDIR=/tmp APP_FEATURES__ENABLE_METRICS=true APP_FEATURES__RATE_LIMIT=5000 go run ./examples/basic