Docs, migration guide, and 2.0 release

[UltraBob]

Documentation, migration guide, and release preparation for 2.0.

Work items

• README for both site and contrib workflows
• Remove line recommending ddev-drupal-contrib for contrib work
• Migration guide for ddev-module-developer users (command renames, container rebuild)
• Retire ddev-module-developer with redirect and credit
• Update add-on registry entry
• Decide governance before 2.0 tag
• Branch swap: 2.x becomes main, current main becomes 1.x

Acceptance criteria

• README documents both workflows
• Migration guide published
• ddev-module-developer redirects to merged add-on
• Registry entry updated
• Governance decided
• Branch swap complete

Blocked by: All implementation epics