Add `next` branch for major version development

Update branch policy documentation to introduce a three-tier branching
strategy where the next branch is used for major version updates, main
for minor updates, and x.y-maintenance branches for patch releases.

This change clarifies the development workflow by providing separate
branches for breaking changes (next) and new features (main), while
maintaining backward compatibility through the existing maintenance
branch structure.

Co-Authored-By: Claude <noreply@anthropic.com>