Add categories to topics

[adrienpoly]

Given that the list of topics will grow to several hundreds of topics I think it could be good to fit them into some main categories

Here is my first stab at defining this list

• Ruby Language, Frameworks & Gems
• Development Practices & Tools
• Architecture & Design
• Frontend & UI/UX
• Infrastructure & Performance
• Community, Career & Personal Development

Rather that a simple filter by letters for topics we could have a first level of filter by main categories

can it be a simple category enum on the topic or does it needs to be a dedicated model with a many to many association

I think a simple enum would be good enough but happy to change my mind