feat: Admin can create/edit MembershipTemplates

[LobeTia]

🔗 Linked issue

Resolve #27

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

MembershipTemplates are required in order to give the user multiple ways to subscribe to an association
SO:
Added a new admin section /membership/manage where users can manage the MembershipTemplates
Layout is handled by Next.js parallel routes + intercepting routes
^ This is potentially overkill but serves as a working example on how to handle modal+full page duality in features inside the project
Future PRs will rollout the template to the rest of the project (onboarding, members portal) and adapt the database

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.