docs(pii): `data_collection` docs, onboarding snippet & TODO cleanup

[sl0thentr0py]

Stack position: 10/10. Blocked by #3003 … #3007**.**

Ship the user-facing surface once wiring is complete.

Scope

• Update user-facing SDK docs (sentry-docs) and the develop-docs Ruby section for data_collection and the send_default_pii deprecation/mapping.
• Add the commented opt-out data_collection snippet (à la the JS reference) to all Ruby onboarding / README / sample Sentry.init blocks — including spec/apps/rails-mini/app.rb and each gem README.
• Grep for and resolve any remaining send_default_pii-related TODOs.

Notes

• Document the stack_frame_variables → include_local_variables and frame_context_lines → context_lines mapping.
• Cross-link the deprecation notice to the docs.

[linear-code]

RUBY-197