docs: add SDK regeneration and release workflow

[thom-techlete]

Summary

Document the standard workflow for regenerating the Zero Friction Python SDK from updated OpenAPI specifications.

This adds guidance for:

• branch-first SDK regeneration instead of regenerating directly on main;
• keeping OpenAPI Generator pinned to the currently used version (7.13.0);
• regenerating one client or all eight generated clients;
• reviewing generated diffs for SDK-surface changes;
• validating the regenerated SDK against dependent repositories before merge;
• tagging the current SDK as v1 and the next validated regeneration as v1.1 after merge;
• preferring release tags over main for stable downstream installs.

Files

• docs/8_sdk_generation.md
• README.md
• CONTRIBUTING.md

This PR only changes documentation; it does not regenerate the SDK itself.