[Member codes] Update documentation for member codes project

[elichad]

• project page in docs
• database structure pages according to changes in models
• admin guide including screenshots for TR/WRs
• the soft validation implementation, a new design pattern which we may reuse later - it can be added to https://carpentries.github.io/amy/design/design_patterns/
• check for extra manual deployment steps

Created automatically from an Asana task.