docs: add maintenance guide for stage configuration updates

[forkfury]

Fix

Replaces TODO comment with maintenance documentation for StageConfig to help developers identify all locations that need updates when adding or removing pipeline stages.

Changes

• Removed TODO comment from Config::stages field
• Added cross-reference to StageConfig documentation in Config struct
• Added maintenance guide in StageConfig documentation listing:
  • Required struct field updates
  • StageId enum updates
  • Stage set builder updates
  • Documentation updates
  • Test updates